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!
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.