An unreasonable or impossible demand; an extremely difficult or unfair problem/request.
客が無理難題を押しつけてきて、対応に困った。
A client imposed unreasonable demands, and we didn't know how to handle them.
新人にそんな無理難題を頼むのは酷だ。
It's harsh to ask a newcomer to do something so impossible.
交渉で無理難題を出されたら、冷静に代替案を示そう。
If faced with unreasonable demands in negotiations, calmly propose alternatives.