metaphor(ic) Vs idiom(atic)

I would like to understand what differences and relations are between metaphor and idiom, and between metaphoric and idiomatic. Following are quotes from me asking two people on this site:

zpletan said after this answer:

I would say that something smelling fishy is a metaphor that has become idiomatic. But maybe I'm wrong. :) Metaphor is "a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable," while idiom is "a group of words established by usage as having a meaning not deducible from those of the individual words." (New Oxford English Dictionary)

Nico said in one comment following his reply:

idiomatic means: Peculiar to or characteristic of a particular language (OED). So you could have an idiomatic English metaphor/simile/allegory etc. as opposed to a French or Spanish one. However idiomatic is not restricted to figures of speech, you can say that putting the verb at the end of the sentence is idiomatic of German.

It looks like different people have different opinions. So are there consensus opinion on this question?

Solution 1:

Both are correct. A metaphor is, as stated, applying a word where it does not literally belong. "Something rotten in the state of Denmark" (Hamlet) means something decaying, or just plain full of maggots, which describe neither kings nor policies in the strict sense; but a translator could use the word for rotting in his language without loss of meaning. (By the way, the usual adjective is metaphorical.)

But that quotation is so familiar in English that a journalist could say "it smells like the state of Denmark" and be understood by most of his audience. He would be using an idiom comprehensible to those who know the play, but those who don't could not possibly deduce the meaning. A translator would have to find a similar allusion in his language, or risk offending the Danish parliament without conveying the sense. You could call this a Shakespearean idiom, or an English idiom, equally.

Similarly, if you the verb at the end of the sentence put, you are using a German idiom. A German student could justifiably say "But all the words are correct, and good authorities say the position of the verb is unimportant as long as it is not ambiguous: how can you say it is wrong?" The answer is simply that it is unidiomatic in English.

Solution 2:

The two comments are not really conflicted.

You need to take into account though that both idiom and metaphor have a more strict and more relaxed definitions.

Idiom

1. a group of words established by usage as having a meaning not deducible from those of the individual words (e.g. see the light).
2. a form of expression natural to a language, person, or group of people.

Metaphor

1. a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable
2. a thing regarded as representative or symbolic of something else

For metaphor wikipedia states that in broader sense "antithesis, hyperbole, metonymy and simile would all be considered types of metaphor".

Built on top of the basic terms the comments talk about

• metaphors that become idiomatic
• idiomatic metaphors

Metaphors that become idiomatic

One good example here is "silver lining", the metaphor is from a poem and is intuitively understandable. It is also a nice phrase and it took hold in the language. The expression through use acquired and fixed its meaning and now it is an idiom.

A different metaphor that become idiomatic is chip. Originally chip was O.E. cipp and meant only "piece of wood,". Since 1769 it is used for "thin slices of foodstuffs (originally fruit)."
However, this did not happen over night. When new meanings of the words develop the words are necessarily used¹metaphorically (most slang is metaphorical in some way). What ended up idiomatic in this development is that this meaning was kept in BrE and not in AmE and this sense of chip became idiomatic for BrE. (The other meaning of "counter used in a game of chance", first recorded 1840 is idiomatic in AmE)

Idiomatic metaphors

For example if "chip" in BrE sense was used in a poem as part of another metaphor, let's imagine a line "last of my last chips I would share with you", here there is metaphor at play, but understanding of it depends on the audience, hence you can call it idiomatic metaphor according to the 2nd meaning of idiomatic ("a form of expression natural to a language..."). Another example could be "my bucket's kicker" as metaphor for "my killer"; here an idiom is used to create a metaphor.

Also, this could mean a metaphor which is well known - for example time is money; here the metaphor has became so well known that the expression is set in its form and acquired proverbial status.

¹ Used here means both "said as" and "understood as".

Solution 3:

For most people, an idiom is an expression where the meaning is not immediately apparent from a literal interpretation of the words.

A metaphor is a more extreme form of a simile. A simile is a comparison made between A and B, and a metaphor is where you say A actually is B, even though that's not literally true.

Idiom: The path takes a dog leg - The path bends like a male dog's leg when urinating.

Simile: She's like a bitch on heat - She's keen to have sex, like said bitch.

Metaphor: She's a bitch on heat - As above, but she is directly equated with bitch on heat.

In principle, one could thus say that all metaphors are idioms, in that the intended meaning isn't actually the literal meaning. But where the intended meaning is reasonably obvious, most people wouldn't normally call it an idiom.

Also note that idiomatic usage often refers to cases where there's no obvious reason why one form of words is consistently chosen over another, even where the meaning would be clear in either case. We always say hammer and chisel, for example, never chisel and hammer.

EDIT: To address OP's specific question "Is there a consensus opinion on the definitions of metaphor and idiom?", the answer is "No". Though I personally would never say some usage is idiomatic of German. Nor would anyone else - apart from this question itself, that particular word sequence has only three hits on Google, none of which seem to be from a native English speaker.

Solution 4:

A metaphor is one of the most common literary devices, in which one thing is referred to as another in order to indicate similarity.

My heart is an ocean is a (bad) metaphor. My heart, while not literally a great body of water, is deep/expansive/salty/what-have-you.

My heart is an ocean is not, however, an idiom. To be an idiom, it would have to be a common expression, understandable based on prior exposure to the phrase rather than logical parsing of the words.

If you describe a relationship as being on the rocks, you are using an idiom (and not a metaphor). Without knowing the phrase and how it is used, a person would have difficulty understanding that "on the rocks" means "in trouble".

Some metaphors are used idiomatically. Carrot and stick, for instance, is a metaphor that has become idiomatic.