鞭长莫及

Beyond one's grasp.

Out of reach.

eg：I cannot help him because it is beyond my grasp.

中英俚语俗语来源：左传：“虽鞭之长，不及马腹。”

同义词：远水不救近火 Water afar off quencheth not fire.

反义词：信手拈来 Come in handy.