not know own strength

not know (one's) own strength

To underestimate how much force one is able to apply or exert in some physical activity. A: "Hey, you're pushing the car too fast! I just need to get it off the road, not down into the ditch!" B: "Oops, sorry. Guess I don't know my own strength!" You need to be more careful with that equipment. You don't know your own strength!
See also: know, not, own, strength
Farlex Dictionary of Idioms.

not know one's own strength

not to realize how destructive or harmful one's strength can be. I didn't mean to hurt you. I guess I don't know my own strength. He might break the door down by accident. He doesn't know his own strength and could end up pushing too hard against the door.
See also: know, not, own, strength
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • not know (one's) own strength
  • backbone
  • bring it weak
  • bust (one's) butt
  • bust butt
  • bust butt to do something
  • bust one’s butt
  • bust one’s butt to do something
  • exert
  • exert (oneself)