adverb
about 80%; mostly; almost certainly
Used to indicate that something is roughly 80% complete, or that a situation is largely settled. The 'almost certainly' sense is a figurative extension of the quantitative meaning.
工事は八割方終わっている。
The construction is about 80% finished.
彼が来るのは八割方間違いない。
It's almost certain that he will come.
Compound of 八割 (80%) + 方 (direction, side, approximately). The 方 here functions as a suffix meaning 'about' or 'in the range of'.