An idiom meaning 'in eight or nine cases out of ten'; very likely; in all probability.
彼は忘れ物が多いから、十中八九今日は遅刻するだろう。
He often forgets things, so in all probability he'll be late today.
十中八九この計画は承認されるはずだ。
This plan will very likely be approved.
十中八九雨が降るから、傘を持って行った方がいいよ。
It's very likely to rain, so you should take an umbrella.