expression, i-adjective
ill-bred; poorly brought up
Used to describe someone whose upbringing is considered lacking, resulting in bad manners or poor character. Often carries a judgmental tone.
彼は育ちが悪いから、ああいう失礼なことを平気で言うんだ。
He's ill-bred, so he says rude things like that without a second thought.
育
If you're poorly brought up, you'll struggle when you enter society.
Compound of 育ち (upbringing) + が (subject marker) + 悪い (bad). The phrase is a straightforward description of poor upbringing.