不自量力
Go beyond one's depth.
Overreach oneself.
Bite off more than one can chew.
Throw straws against the wind.

eg：Trying to solve that problem is to 90 beyond her depth for Lily.

中英俚语俗语来源：战国策·齐策三：“荆甚固，而薛亦不量其力。”

同义词：瞠臂当车 Kick against the pricks.
反义词：人贵有自知之明 Know myself.