A group of words which have a different meaning when used together from the one they would have if you took the meaning of each word separately.
not know one's own strength
to not realize how destructive or harmful one's strength can be
The little boy does not know his own strength and does much damage when he plays.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.