〜だに
even (just) thinking; not even
Verb dictionary form (考える, 想像する, 聞く) + だに · Noun + だに + negative
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だに is a literary "even" with two uses.
After a verb of thinking or hearing, it means "just doing that is enough": 考えるだに恐ろしい, "it's terrifying even to think about". The verbs are few (考える, 想像する, 聞く), and the second half is a strong feeling like 恐ろしい or ぞっとする. The everyday version is だけで.
After a noun with a negative, it means "not even": 夢にだに思わなかった, "I never even dreamt of it". 微動だにしない, "doesn't budge an inch", is a fixed phrase.
It's old-fashioned and appears mostly in fixed expressions and literary writing. Compare すら and さえ, which are more flexible.
Examples
考えるだに恐ろしい。
It's terrifying even to think about.
想像するだにぞっとする。
Just imagining it gives me the shivers.
こんな日が来るとは、夢にだに思わなかった。
I never even dreamt this day would come.
兵士たちは微動だにしなかった。
The soldiers didn't budge an inch.
聞くだに恐ろしい話だ。
It's a story that's frightening just to hear.