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.
get (someone or something) out of one's mind/head
to forget about someone or something
It took me several months to get my old girlfriend out of my mind.
List of common idioms, their meanings and examples of their usage in sentences
Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.