not know (one's) own strength(redirected from didn't know their 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!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.