An adverb meaning very much, a great deal, or a lot (colloquial).
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.
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.
うんと待ったけど、結局来なかった。
I waited a long time, but in the end they didn't come.
そのケーキはうんと甘いので、一切れで十分だ。
That cake is very sweet, so one slice is enough.
An archaic/mimetic adverb meaning to do something with great effort or force.
二人はうんと力を合わせて門を押し開けた（古い表現）。
The two of them put forth a great effort and pushed the gate open (old-fashioned expression).