"Skycap" is probably related to "redcap" -- a railway porter (who usually wore a red cap).
A skycap is a porter employed by an airport and provides the following service to airline passengers:
. Perform curb side check-in
. Pushing wheelchairs
. Generally works for tips
The word is a portmanteau of the words sky and redcap (a rail station porter).
A Skycap's services are popular among travelers who carry excess baggage, such as news photographers and professional athletes.
(Oxford English Dictionary)
Chiefly U.S. A railway porter.
1911 Chicago Defender 3 June 1/7 It is the consensus of opinion that if the colored men prove efficient, the many other stations will willingly install colored ‘red caps’.
1931 W. FAULKNER Sanctuary xxi. 227 A man shouted ‘taxi’ at them; a redcap tried to take Fonzo's bag.
skycap N. Amer. [after REDCAP 5], a porter at an airport
1950 Official Gaz. (U.S. Patent Office) 26 Dec. 1066/2 *Sky Cap. For General Porter Service.
1966 National Observer (U.S.) 7 Nov. 6/5 They would reduce the number of Negro ‘sky caps’ employed at the airport.
1972 T. KENRICK Tough One to Lose ii. 33 He took a job as a skycap at the International Airport.
24 December 1940, New York (NY) Herald Tribune, pg. 11, col. 8:
Air-Line Porters "Skycaps"
Willie Wainwright, of New Orleans, was announced yesterday as the winner of a competition to select a name for porters at the Airlines Terminal, Forty-second Street and Park Avenue. His suggestion was "skycaps," and he won $100. There were 2,780 persons in the contest.