The idiom "one of the boys" refers to someone included in a specific social group. It generally applies to inclusion in a groups of males or in a group traditionally dominated by males. The term originally meant an ordinary, amiable man, with the first recorded use occurring in 1893.
An example of usage of the phrase is, "Mary always went out for drinks after work with us. She was just one of the boys." The phrase may also denote the inclusion of nonhumans, such as in the sentence "the dog thought he was one of the boys."
Idioms are phrases that have a set meaning within a culture, either literal or figurative. Most speakers use the phrase "one of the boys" literally, although it appears figuratively when the word "boys" indicates nonhuman or nonmale entities.