Shakespeare's “say sooth” vs. “tell truth”

The noun sooth, pronounced /suːθ/, is now archaic and means ‘fact’,‘reality’ and ‘truth’. Its legacy persists in the words soothe /suːð/, and soothsayer meaning someone who sees the truth, a synonym of fortune teller and the French loanword clairvoyant.

In Shakespeare's plays, sooth is often used with the verb say and in the expression in sooth whereas truth is often used with the verb tell

From Shakespeare's Macbeth

Sergeant:
If I say sooth, I must report they were
As cannons overcharged with double cracks, so they
Doubly redoubled strokes upon the foe:

(Act I, Scene 2)

Banquo:
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray’s
In deepest consequence.

(Act I, Scene 3)

Macbeth:
[Aside] Two truths are told,
As happy prologues to the swelling act
Of the imperial theme.—I thank you, gentlemen.

(Act I, Scene 3)

Macbeth:
If thou speak'st false,
Upon the next tree shalt thou hang alive,
Till famine cling thee: if thy speech be sooth,
I care not if thou dost for me as much.
I pull in resolution, and begin
To doubt the equivocation of the fiend
That lies like truth: 'Fear not, till Birnam wood
Do come to Dunsinane:'…

(Act V, Scene 5)

Merchant of Venice

Antonio:
In sooth, I know not why I am so sad:
It wearies me; you say it wearies you;

(Act I, Scene 1)

Etymonline tells me that truth meaning "something that is true" was first recorded in the mid-14 century. In the 1560s, truth came to mean "accuracy, correctness". Meanwhile, the noun sooth is dated 900 and was derived from soð.

• I would like to know what difference in meaning, if any, was there between sooth and truth?
• Why did “sooth” become obsolete, and when was it eventually overtaken by “truth”?

Solution 1:

World Wide Words appears to suggest that ther is no real difference in meaning between sooth and truth:

Sooth does indeed mean “truth”, an Old English word. It has not been in daily use for about four centuries, except in the phrases by my sooth or my sooth, interjections now obsolete which emphasised that the speaker was telling the truth.

Regarding your second question, it appears the term was commonly used till mid 1600s and has been considered archaic since then.

Sooth:

Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). In common use until mid-17c., then obsolete until revived as an archaism early 19c. by Scott, etc. Used for Latin pro- in translating compounds into Old English, e.g. soðtacen "prodigy," soðfylgan "prosequi."

(Etymonline)

Sooth was reintroduced in the nineteenth century as a literary archaism by writers such as Sir Walter Scott.

• In sooth, there was that in her face and in her voice when she spoke which almost made Anne weep, through its strange sweetness and radiance. (A Lady of Quality, by Frances Hodgson Burnett, 1896. This work is exceptionally full of sooth — the author uses the word 20 times.)

Solution 2:

Etymonline says sooth -- born with the English language itself -- was in use till mid 17th century, but it doesn't directly explain its demise. (I liked the bit about it being connected to sin as in “being the one”, which makes sin a relative of the modern day German verb 'to be', sein).

It seems there must have been a long phase where 'truth' and 'sooth' performed different functions. 'Truth', also around since Old English days, originally had the sense of being true as in demonstrating faithfulness and loyalty.

From dictionary etymology

Old English triewð (West Saxon), treowð (Mercian) "faith, faithfulness, fidelity, loyalty; veracity, quality of being true; pledge, covenant," from Germanic abstract noun *treuwitho, from Proto-Germanic treuwaz "having or characterized by good faith," from PIE *drew-o-, a suffixed form of the root *deru- "be firm, solid, steadfast." With Germanic abstract noun suffix *-itho (see -th (2)).

Until the mid 14th century 'sooth' alone would have covered the other uses 'truth' has today of expressing factuality.

Old English soð "truth, justice, righteousness, rectitude; reality, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth").

But it would seem then that in the mid 14th century the meaning of truth expanded. (still etymology online)

Sense of "something that is true" is first recorded mid-14c. Meaning "accuracy, correctness" is from 1560s.

According to Wikipedia, modern day English 'truth' is now alone amongst Germanic languages.

All Germanic languages besides English have introduced a terminological distinction between truth "fidelity" and truth "factuality". To express "factuality", North Germanic opted for nouns derived from sanna* "to assert, affirm", while continental West Germanic (German and Dutch) opted for continuations of wâra "faith, trust, pact" (cognate to Slavic věra "(religious) faith", but influenced by Latin verus). Romance languages use terms following the Latin veritas, while the Greek aletheia, Russian pravda and South Slavic istina have separate etymological origins.

[*cognate with soð according to etymonline]

But it seems to me that it is not so much the case that English didn't “introduce a terminologicad distinction”, but rathermore that it discarded the one it had.

I'm afraid I didn't find a direct explanation as to why sooth fell out of use, but if wikipedia and the online etymology dictionary speak sooth then it seems that 'truth' elbowed its way into 'sooth' territory in the mid 14th century. It's pure conjecture, but perhaps this contributed to the demise of 'sooth' in mid 17th century; it seems natural enough to assume that since 'truth' covered sooth and more, then if one were to fall by the wayside it would be 'sooth'. Strewth!