Onomatopoeic adverb meaning to be completely beaten, crushed, or defeated; thoroughly/utterly (often colloquial).
昨日の試合でチームはこてんぱんに負けた。
The team was completely beaten in yesterday's match.
彼は議論でコテンパンにやられて黙ってしまった。
He was utterly routed in the debate and fell silent.
宿題を忘れて先生にこてんぱんに怒られた。
I forgot my homework and was scolded severely by the teacher.