Imbecilic is the adjective for something that seems extremely stupid, like the behaviour or ideas of an imbecile. People say an imbecilic remark, an imbecilic plan, or imbecilic behaviour. It is not the person-word; imbecile is the person (and can also act like an adjective). Imbecilic is more clearly “of that stupid kind.” It is rude and rather bookish. You will meet it in writing more than in friendly talk. Though actually, because it comes from an old medical insult, it still carries a nasty taste. Use it only when you are describing the stupidity of an act or idea, and even then, know that it lands hard.

愚蠢的；白痴似的（語氣很重）