An adverb meaning 'very', 'considerably', or 'to a large extent' — used to emphasize degree.
彼はよっぽど頭がいいから、すぐ理解した。
He's very smart, so he understood immediately.
今日はよっぽど疲れているのか、早く寝た。
He must be really tired today; he went to bed early.
よっぽど悔しかったのだろう、彼は泣いてしまった。
He must have been extremely frustrated; he ended up crying.
Used to mean 'almost' or 'nearly' (e.g., 'almost did something' or 'was just about to'), indicating an action that nearly happened.
電車に乗り遅れそうで、よっぽど走った。
I almost missed the train, so I ran like mad.
よっぽど怒っていたのか、席を立って出て行った。
He must have been about to get really angry — he stood up and left.
事故に遭うところで、よっぽど助かった。
I was almost in an accident; I was really lucky to be saved.