〜くせに
Even though, and yet (reproachful)machine translation
FormationVerb／い-adjective plain form + くせに; な-adjective + なくせに; noun + のくせに
〜くせに means "even though…, still…," always carrying a REPROACHFUL, disdainful tone: 何も知らないくせに偉そうに話す (he knows nothing, yet always talks like he's a big shot). This is a strongly emotional pattern, used only in speech and with equals or people below you — absolutely never use it about a superior to their face.
Two important constraints: both clauses must have the SAME subject, and that subject must be a PERSON (or animal) — you can't use くせに to blame the weather or objects. Connection: noun + のくせに, な-adjective + なくせに, everything else in the plain form.
Compared with のに: both mean "and yet," but のに is neutral, merely expressing surprise or regret, while くせに is like pointing a finger in the other person's face. 知っているのに教えてくれない (they know but won't tell me — a bit sad); 知っているくせに教えてくれない (they clearly know and are hiding it — reproachful). In formal writing, don't use くせに; switch to のに or ながらも.
Examples
彼は何も知らないくせに、いつも偉そうに話します。
He knows nothing, yet he always talks like he's a big shot.
弟はお金がないくせに、高い服ばかり買っています。
My little brother has no money, yet he keeps buying expensive clothes.
自分でやると言ったくせに、全然やっていませんね。
You said you'd do it yourself, yet you haven't done a thing.