Meaning: やる気のない人に何を言っても無駄だ。
For Example:
‣ I bought all kinds of toys for my cat, but she was only interested in the wrappings. Well, I guess you can lead a horse to water, but you can't make him drink.
猫ちゃんの為にいろんなおもちゃを買ったのに、周りのラッピングの部分にしか興味を示さんかった。まぁ、やる気のない子に何をやってもあかんっていうしね。
‣ The teacher tried his best to educate the student, but she still doesn't do any of the homework. You can lead a horse to water, but you can't make him drink.
先生はその生徒を一生懸命教えたが、彼女はまだ宿題を一切やらないままだった。やる気のない子に何を言うてもあかんとはこのことやね。
Synonym:
You can offer good things to people, but you can't force them to accept that.
People will do whatever they want no matter what others tell them.