Colloquial exclamation of anger, irritation, or disgust. Often used as a sharp, dismissive sound.
けっ、もう知らないよ。
Ugh, I don't care anymore.
けっ、また負けたか。
Tsk, I lost again.
Mimetic interjection; the exact origin is uncertain, but it likely imitates a sharp exhalation or clicking sound of annoyance.