Different words for "idiom"


When looking on dictionaries E-J for the word "idiom" in Japanese, the
following results appear: 熟語、慣用句、成句、故事成語、ことわざ and 言い習わし.

While I think ことわざ is more like a "proverb", what about the rest? Which would
be the difference between them?

Could you please give me an example or some with each category
(熟語、慣用句、成句、故事成語、ことわざ and 言い習わし) of idioms?

In which category would you put things like 朝飯前、二階から目薬, 馬の耳に念仏 or 雲泥の差?




**熟語** is, in a broader sense, everything that is a compound word, especially
which has opaque (not predictable from each part) meaning among them. However,
it most often refers to any type of multi-kanji compound, such as: 熟語 (itself
二字熟語) or 弱肉強食 (四字熟語).

**慣用句** is a set phrase, especially those with opaque meaning, such as: 足を洗う
("wash one's feet", but figuratively, to retire from bad deeds, where English
would use "wash one's hands")

**故事成語** is an imported idiom that has reference (故事) to the Chinese classics,
most of which are 熟語. For example, 四面楚歌 is from [this story in the
chronicles](https://en.wiktionary.org/wiki/%E5%9B%9B%E9%9D%A2%E6%A5%9A%E6%AD%8C),
and 矛盾 from [a passage from Han
Feizi](https://en.wiktionary.org/wiki/%E7%9F%9B%E7%9B%BE).

**ことわざ** is a type of 慣用句 that I think best translated _proverb_ , a
figurative expression which usually conveys some folk wisdom. What I typically
imagine from this word is those which can stand as a full sentence in the
form. For example, 猿も木から落ちる "even monkeys fall from trees" or "even Homer
sometimes nods".

成句 and 言い習わし don't have a good definition in my opinion. 成句 can be a bookish
alternative for either 熟語, 慣用句, or ことわざ in my definitions, and 言い習わし is a
verbal noun out of 言い習わす "say customarily/traditionally", which means what it
does.

So, 朝飯前 would fall under 熟語; 二階から目薬 and 馬の耳に念仏 are 慣用句 as well as ことわざ; 雲泥の差
is both 慣用句 and 故事成語 (and maybe 熟語).

