Green’s Dictionary of Slang

(US) a female telephone operator.

1889		‘Mark Twain’ Connecticut Yankee xv 177: The humblest hello-girl along ten thousand miles of wire.
1897		Dalles Dly Chron. (OR) 1 Sept. 3/4: Here the hello-girl switched us off.
1905		Sporting Times 11 Mar. 2/3: Many, many years ago I fell enamoured of a fuzzy siren of a hello-girl. I used to ring her up and hold her line for lengthened periods while I fixed it up that we might revolve for a few hours in the same orbit when the lamps were lighted and she came off duty.
1910		Herald & News (Neberry, SC) 5 Aug. 5/2: The helo girls’ wages range from $22.40 per month Nashville, Tenn. to $36.96 in New York City.
1915		Day Book (Chicago) 9 July 10/1: Some day we are going to organize the telephone girls of Chicago. Every hello girl in the city will be in the union.
1921		N.Y. Tribune 14 Aug. 60/2: The heroine is a hello girl.
1948		Chicago Daily News 27 Aug. 10/6: [heading] Hello Girls Seek HCL Raise Here [DA].