adverb
Expresses that something is surprisingly or excessively much; often used with a negative or surprised tone.
今日はいやに暑いね。
It's awfully hot today, isn't it?
彼はいやに静かだ。
He is terribly quiet.
Derived from the adjective 嫌 (iya, 'unpleasant') + adverbial particle に (ni), originally meaning 'in an unpleasant manner', later shifting to an intensifier.