1Sly, sneaky, or unfair — acting dishonestly or taking advantage of a situation for one's own benefit. Common in everyday speech, including kids complaining "that's not fair!"
試験でカンニングするなんてずるいよ。
Cheating on an exam is unfair.
彼はずるい手を使ってゲームに勝った。
He used a sneaky trick to win the game.
自分だけ先におやつを食べるなんてずるいよ!
It's not fair that you got to eat the snack first!
ずるいやり方で成功しても尊敬は得られない。
Even if you succeed by unfair means, you won't earn respect.
2Used with the reading "kosui": miserly or stingy, describing someone unwilling to spend or share money. A somewhat old-fashioned/dialectal usage (associated with Kansai speech).
彼はいつも割り勘にするので、少しこすいところがある。
He always splits the bill evenly, so he's a bit stingy.
お金に細かくてこすいと言われることがある。
He's sometimes called stingy because he's so particular about money.