Examples:
- うつ病を経験したことのない人に、そのつらさは分かりっこない。People who have never experienced depression do not know how hard it is.
- 今から二日でこの本の単語を全部覚えろなんて、できっこないよ。I can't remember all the words in this book in the next two days.
- 一日十時間も勉強するなんて、私にはできっこないよ。I can't study for ten hours a day.
Generally used in spoken language, it means to strongly deny the possibility of something happening.