A “barfly” (also “bar fly”) is someone who spends a lot of time in a bar—often waiting for someone to arrive to pay for the drinks. “Bar fly” has been cited in print since at least 1890.
 
   
Merriam-Webster’s Online Dictionary
bar·fly noun \ˈbär-ˌflī\
: a person who spends a lot of time drinking in bars
First Known Use of BARFLY
1910
 
28 November 1890, Cincinnati (OH) Commercial Gazette, pg. 1, col. 5:
He was a hard drinker, a bar fly, but was considered a harmless fellow.
 
13 December 1895, Kansas City (MO) Times, pg. 4, col. 6:
“TWAS A COSTLY DRINK.
William Tracey Swallowed Another’s Beer and Was Sent to the Work-House.
A stranger called at the saloon of W. K. Clark in the West bottoms Wednesday and ordered a glass of beer. William Tracey, a bar fly, was hanging around and he boldly stepped up and drank the foaming brew.
 
4 August 1904, Kansas City (MO) Star, “Bishop Potter’s Model Dramshop,” pg. 6, col. 2:
It is a foregone conclusion, to begin with, that there will be no “old soaks” or “barflies” around Bishop Potter’s model dramshop.
 
Chronicling America
28 June 1905, The Evening Star (Washington, DC), pg. 8, col. 6:
Soda Fountain “Barflies,”
From the Kansas City Druggist.
Saloons have their “barflies,” but in that respect they are not different from stores to which there are soda fountains. There is a difference in “barflies,” though. A “barfly” in a saloon is a man who hangs around and waits to be invited to drink free. A soda fountain “barfly” is a girl or a woman who sits around waiting for her friends to drop in and invite her to have soda or ice cream.
 
Google Books
The Big Rock Candy Mountain
By Wallace Stegner
New York, NY: Doubleday
1938
Pg. 81:
This man was obviously no officer, but only a tramp or barfly wandering in on his way through town.
 
Sidewalks of America
Edited by B. A. Botkin
Indianapolis, IN: Bobbs-Merrill
1954
Pg. 515:
Another famous gag of this sort and period took place between a cop and a barfly on Times Square.
   
Wikipedia: Barfly (film) 
Barfly is a 1987 American film which is a semi-autobiography of poet/author Charles Bukowski during the time he spent drinking heavily in Los Angeles. The screenplay by Bukowski was commissioned by the French film director Barbet Schroeder – it was published, with illustrations by the author, in 1984 when film production was still pending. Barfly stars Mickey Rourke and Faye Dunaway, with direction by Schroeder, and was “presented by” Francis Ford Coppola. The movie also features a silent cameo appearance by Bukowski himself.