1Waste, scraps, leftover pieces, or fine debris produced when something is made or used — bits left behind. Most often appears in compounds: 紙くず (paper scraps), 木くず (sawdust/wood shavings), 鉄くず (scrap metal), 野菜くず (vegetable trimmings), パンくず (bread crumbs). The kanji 屑 is non-jōyō, so hiragana くず is standard in modern writing.
床に落ちた紙くずをほうきで掃き集めた。
I swept up the paper scraps that had fallen on the floor.
木工作業のあとは木くずがあちこちに散らばっていた。
After the woodworking, sawdust was scattered all over.
野菜くずはコンポストに入れて肥料にする。
Vegetable trimmings go in the compost to become fertilizer.
鉄くずは業者に引き取ってもらえる。
Scrap iron can be taken away by a recycler.
パンくずを揚げ物の衣として使う。
Bread crumbs are used as a coating for fried foods.
2(Slang, derogatory) A harsh insult labeling someone as worthless, despicable, or 'the lowest of the low.' Common in compounds like クズ男 (a scummy guy) and クズ人間 (a piece of human trash), and standalone in lines like あいつはクズだ. This is a strong slur — using it can lead to serious conflict. Modern Japanese writes it in katakana クズ to signal the slang/insulting register.
あんなふうに人を裏切るなんて、あいつはほんとうにクズだ。
Betraying people like that — that guy is a real piece of garbage.
彼女が別れた相手は、典型的なクズ男だったらしい。
Apparently the guy she broke up with was a textbook scumbag.
他人を傷つけて平気な人間はクズだと思う。
I think anyone who can hurt others without remorse is scum.