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.
lock horns with (someone)
to get into an argument with someone
I locked horns with a woman who I work with when we had an argument last week.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.