Idioms must be the bane of people trying to learn English as a second language. (These days, even native-born Americans can have a hard time keeping up with them.)
But imagine a foreign English-learner forced to deal with a phrase like "pull someone's leg." He or she must be taught what it means, or they are out of luck. Supposed they learn it, only to hear the next day someone tell a friend heading into a difficult test of some kind to "break a leg."
Some idioms can be figured out. "Head over heels in love" doesn't make a lot of sense, but you get the idea. To "spill one's guts" isn't especially lucid, still, in context, an English student can figure it out.
What, however, about those archaic phrases that all English speakers know, even if they no longer know what the words mean? I'm thinking of idioms like "kit and caboodle." Ask any middle-school kid what it means, and they'll know - call it the "whole ball of wax," to use another idiom. ("Kit" refers to all the necessary things a soldier takes with him in his kit; caboodle harkens back to "boodle" - a gambler's entire stake - and probably became "caboodle" to be an alliteration with kit."
But some idioms reach a certain level of poetry, however unconsciously. For instance, to "leave high and dry" would seem to involve escaping a flood - hardly the meaning. To "kick the bucket" wouldn't immediately evoke the gruesome sight of someone kicking out in one last death throe. You have to make the connection yourself.
One of the idioms I find most interesting can be attributed to some relatively affluent girl in some California suburb in the 1980s. It is this: "Gag me with a spoon." It may be deservedly disappearing, but I still can't hear it with thinking about spoiled adolescents and their silver spoon. Is this just a combination of idioms? Sure. Is the connection just taking place in my head? Of course. But the resonance of Valley Girls and the silver spoon they were born with won't go away. Isn't that what poetry is?