In modern English, the idiom is used as an exaggerated greeting or as a description of a personality type. Modern use of the term tends to be deliberately archaic, with overtones of over-familiarity in the person (almost always male) so described, or as a deliberate, tongue in cheek term of endearment, heightening the effect of the greeting of an unexpected friend when as in "the only friendly person here" or to remark that there is a friend in an unfriendly atmosphere. It can also be used as a pejorative as a description of a person rather than a greeting . The OED gives an example from the 1600s of the use of "hail fellow" as a description in a similarly pejorative manner.
It is also cited as "heartily friendly and congenial, comradely, hail-fellow - characteristic of or befitting a friend; 'friendly advice'; 'a friendly neighborhood'; 'the only friendly person here'; 'a friendly host and hostess'." (See FreeDictionary.com listing below.)