An adverb meaning 'awfully' or 'unusually' (indicating a surprisingly large degree or oddness).
今日はいやに寒いね。
It's awfully cold today, isn't it?
彼は会議でいやに静かだった。
He was unusually quiet during the meeting.
このケーキ、いやに甘いね。
This cake is terribly sweet, huh.