An idiom is an expression that takes on a figurative meaning when certain words are combined, which is different from the literal definition of the individual words.
Every language has its own expressions and idioms, very common for native speakers. But if you are learning a foreign language, I’m sure some of these sentences will sound really funny (and a little bit crazy) to you.
Let me begin with some English idioms to say you are out of your mind: go nuts, go bananas, be a basket case, have bats in the belfry, lose one’s marbles or my favorite, have rats in the attic.
Italians will use the expressions fuori di testa (out of your mind), essere pazzo come un cavallo (be crazy as a horse) or essere fuori come un balcone (be outside as a balcony).
In German, there are two idioms that I personally find really original, Du hast nicht alle Tassen im Schrank (You don’t have all your cups in the cupboard) and Du hast einen Vogel (you have a bird). The first time I heard this last one, I began looking for a real bird in the room.
And in Spanish we normally use estar como una cabra (to be like a goat), estar como una regadera (to be like a watering can) o faltar un tornillo (be missing a screw).
I’m sure your language has also a lot of idioms that will sound crazy to a language student.
Let me know if you want to share them.