To underestimate, belittle, or look down on someone; a transitive verb implying contempt or underestimation.
彼を見くびっていたが、実はとても有能だった。
I had underestimated him, but he was actually very capable.
相手を見くびると痛い目を見ることになる。
If you look down on your opponent, you'll suffer the consequences.
年下だからといって見くびってはいけない。
Don't underestimate someone just because they're younger.