1A colloquial adverb meaning "very much," "a great deal," or "a lot" — used to emphasize a large amount or high degree, similar to すごく or たくさん. Common in everyday spoken Japanese.
今日はうんと寒いから、コートを着て行きなさい。
It's really cold today, so put on a coat before you go.
子供の頃、うんと叱られたことがある。
When I was a kid, I once got scolded really badly.
彼はうんとお金を貯めて新しい店を開いた。
He saved up a lot of money and opened a new shop.
もっとうんと勉強しないと、試験に受からないよ。
If you don't study a lot more, you won't pass the exam.
2An archaic adverbial use meaning "with great force or effort." This sense is essentially obsolete in modern spoken or written Japanese — it survives only as a dictionary entry and is not something learners will encounter in real usage.