What is the difference in meaning between 'nonchalant' and 'insouciant'?
Some background on the difference between 'nonchalant' and 'insouciant'
Before focusing on the meanings of the two words, let's take a look at the Ngram chart for nonchalant (blue line) versus insouciant (red line):
According to Merriam-Webster's Eleventh Collegiate Dictionary, nonchalant entered English (directly from French) circa 1734, whereas insouciance—the noun form of insouciant—arrived (also directly from French) in 1799. The Ngram chart suggests that nonchalant has been several times more common than insouciant in published English texts for the past 150 years or so. Even so, I was somewhat surprised that the Eleventh Collegiate doesn't give insouciant a separate entry. Here are its entries for nonchalance and nonchalant:
nonchalance n (1678) : the state or quality of being nonchalant
nonchalant adj {F[rench] fr[om] pr[esent] p[articiple] of nonchaloir to disregard, fr[om] non- + chaloir to concern, fr[om] L[atin] calēre to be warm —more at LEE} (ca. 1734) : having an air of easy unconcern or indifference syn see COOL — nonchalantly adv
And here is its entry for insouciance:
insouciance n {F[rench], fr[om] in- + soucier to trouble, disturb, fr[om] O[ld] F[rench],fr[om] L[atin] sollicitare — more at SOLICIT} (1799) : lighthearted unconcern : NONCHALANCE — insouciant adj — insouciantly adv
The synonyms note under the entry for cool discusses a group of similar adjectives: cool, composed, collected, unruffled, imperturbable, and nonchalant. Considering that MW views insouciance as meaning "lighthearted unconcern: NONCHALANCE," The exclusion of insouciant from the cluster of synonyms allied with cool is surprising and disappointing. The note on nonchalant in the entry under cool reads as follows:
NONCHALANT stresses an easy coolness of manner or casualness that suggests indifference or unconcern {a nonchalant driver}
This very much of a piece with the discussion of nonchalant in Webster's Dictionary of Synonyms (1942):
Nonchalant stresses an easy coolness of manner, or casualness that suggests, rather than necessarily implies, indifference or unconcern: it often connotes lightheartedness; as, "God...knows, if he is not as indifferent to mortals as the nonchalant deities of Lucretius" (Byron); "Dallying with a cigar, which he smoked nonchalantly as he sang (T. E. Brown); "He walked in a nonchalant fashion" (D. H. Lawrence).
Meriam-Webster's Dictionary of Synonyms (1984) repeats its 42-year-older predecessor's entry almost word for word, except tht it drops the phrase "rather than necessarily implies" and replaces the T.E. Brown quotation with this one:
at the back {of the ambulance}, haughty in white uniform, nonchalant on a narrow seat, was The Doctor—Sinclair Lewis
And yet the entry for nonchalant in Webster's Fifth Collegiate Dictionary (1936/1941) is quite different from the one that appears in Webster's Ninth Collegiate Dictionary (1983), which is identical to the entry (noted above) in the Eleventh Collegiate. Here is the Fifth Collegiate's entry:
nonchalant, adj. ... Lacking in warmth of feeling, enthusiasm, or interest; indifferent; also, Colloq[uial], casual and imperturbable.
The Fifth Collegiate's entry for insouciance, meanwhile, contains this information:
insouciance, n. ... Want of concern; indifference, esp[ecially] as an attitude of mind
In effect, such difference as remains between nonchalant and insouciant in 1941 seems to reflect the difference in their French roots—nonchalance suggesting indifference through a lack of warmth or concern, and insouciance suggesting indifference through an absence of care or troubledness. The meanings are clearly very close, in any case.
One reference work that does provide discussions of both nonchalant and insouciant is S.I. Hayakawa, Choose the Right Word: A Modern Guide to Synonyms (1968). But rather unexpectedly, Hayakawa assigns the two words to different synonym groups: nonchalant lands in a group headed by flippant; and insouciant falls into group under jaunty.
Here are the relevant comments from these two discussions:
flippant, casual, flip, fresh, nonchalant, sassy, smart, wise These words are all all used to describe particular kinds of attitudes, speech, and behavior. ... Unlike [flippant, flip, sassy, and fresh], casual and nonchalant can be neutral or even complimentary in tone. Both words indicate a lack of concern, interest, or excitement: a casual air; a nonchalant approach to business problems. Nonchalant, however, may suggest an attempt to be disciplined or detached: All during the meeting, Mr. Jessup maintained his nonchalant manner, even when the shouting and arguing were at their height.
...
jaunty, chipper, debonair, insouciant These words are used to describe a brisk unworried, self-assured person or the way in which he acts. ... Insouciant translates literally from the French to mean without care; it also suggests gaiety of manner and sophistication: the insouciant young bohemians who travel from on party to another without a thought for tomorrow.
Here again, the distinction between the two words seems to rest primarily on a perceived difference between lack of concern (nonchalant) and lack of care (insouciant). Hayakawa seems to think that the former reflects an attitude while the latter reflects a spirit; I'm not at all persuaded that he is right.
My answers to the poster's specific questions
With regard to the poster's specific questions, I offer these answers:
1. To judge from the foregoing discussions of the meanings of insouciant and nonchalant, the difference between "a nonchalant shrug" and "an insouciant shrug" might be imperceptible to an objective viewer, although we might expect the insouciant shrug to be a bit more vigorous (because it expresses a carefree attitude) than the nonchalant shrug (which is likely to be muted by the shrugger's lack of concern for whatever has prompted the shrug.
2. Ogden Nash seems to be using the words nonchalance and insouciance without a great deal of regard for their overlap in meaning. He is aiming for comic effect and so is happy to use two words where one would convey the essential notion of cucumberish imperturbability.
3. Nash uses nouciance as a ludicrous spelling for nuisance, nothing more. A similar joke spelling for the sake of a rhyme appears in his famous short poem, "If called by a panther/ Don't anther."
4. As for the sentence "He was (nonchalant/insouciant/blithe) about the poor living conditions of the animals in his farm," I wholeheartedly agree with Edwin Ashworth's comment (beneath the question) that none of the three named options is as suitable as indifferent would be and that a change in preposition from about to to would further improve the sentence:
He was indifferent to the poor living conditions of the animals in his farm.
In my view, indifferent indicates a level of coldness toward potential suffering that the other terms—with their emphasis on disposition (blithe), attitude (nonchalant), or spirit (insouciant)—are ill equipped to convey.
It is play on words, and in several ways.
First, he’s combining the two words nonchalance and insouciance into *nouciance, which is really just an unusual spelling of what is supposed to recall nuisance.
Second, he’s chosen these two French imports because they are negatives that have no corresponding positive forms in English, meaning that there is no *chalance to pair with nonchalant nor any *souciance to pair with insouciance.