The colloquial British-English noun brolly is a clipped and altered form of umbrella—the suffix -y, also -ie, is used to form pet names, such as Katy, and familiar diminutives, such as laddie.
The noun brolly originated in university slang, according to the following from The Slang Dictionary, Etymological, Historical, and Anecdotal (London: Chatto and Windus, Publishers, 1874):
Brolly, an umbrella. Term used at both Oxford and Cambridge Universities.
In the slang of the Royal Air Force, brolly has come to denote a parachute, especially in brolly-hop, denoting a jump made with a parachute.
According to The Scotsman (Edinburgh, Midlothian, Scotland) of Monday 16th October 1933, the New Zealand-born British lexicographer Eric Honeywood Partridge (1894-1979) recorded brolly-hop in Slang To-day and Yesterday. With a Short Historical Sketch and Vocabularies of English, American, and Australian (London: George Routledge & Sons Ltd, 1933). I only have access to the Fourth Edition (London: Routledge & Kegan Paul Ltd, 1970) of Slang To-day and Yesterday, in which “a brolly-hop is a parachute jump” occurs in a list of “a few Royal Air Force terms current in 1932”—Eric Partridge writes that this list is “From a 1932 newspaper cutting signed A. D. S.; sent by a friend who omitted to say whence it came”.
The noun brolly hop occurs among other slang terms in the column Barks from a Pup Tent, published in the Visalia Times-Delta (Visalia, California, USA) of Monday 24th May 1943:
Charles J. Hammer Machinery Co.,
915 East Main Street, Visalia, California.
One of the first things you notice when you step out of civilian life and get in the service is the change in language. A “greaseball” is a mechanic and a fellow on mess detail is a “bean jockey.” On board ship “collision mats” are waffles, coffee is “fuel oil.” A “goopher” is a sweet pilot and “daisy cutting” is low flying. A fellow who talks a good flight is a “bunk flyer” and a parachute jump is a “brolly hop.” Sausages and mashed potatoes are “zeppelins in a fog” and “leave hound” is a fellow who makes use of every last minute of leave. So-long for now—I’ve got to catch a little “kip”—sleep to you.
The following is from Britain relives its victory, by John Huxley, published in The Sydney Morning Herald (Sydney, New South Wales, Australia) of Thursday 4th May 1995:
There are more than 1,500 street parties, a sheet of special-issue stamps and commemorative cans of that much-loved meat substitute Spam. Endless reruns on television of Dad’s Army and replays on the radio of Vera Lynn’s greatest hits. Fashion hints on “how to capture that ’40’s flair”.
Even crazy, cut-out-and-keep guides to services slang that offer translations of such phrases as “a bus driver has made a brolly hop into the gravy”. That is, a bomber pilot has been forced to bale out over the Atlantic, don’t you know, old chap? Suddenly, it seems, Britain has gone into a time-warp. A Second World Wartime warp. Once, of course, the accepted wisdom was that proffered by comic character Basil Fawlty: “Whatever you do, don’t mention the war.” But with Victory in Europe Day less than a week away now the nation has been given official clearance to wallow. Shamelessly. In some instance, shamefully.
The noun brolly hop has given rise to the verbal noun brolly-hopping and to the verb brolly-hop:
1-: The verbal noun brolly-hopping—as in the account by Mortimer Durand of his first parachute jump, published in the Daily Express (London, England) of Wednesday 27th June 1934:
First Brolly Hop
A brolly is an umbrella. A hop is a hop. A brolly hop is a hop into space—with a parachute! Brolly-hopping will become a commonplace to the younger generation of today.
2-: The verb brolly-hop—as in the review of “the new British film, One of Our Aircraft is Missing”, published in The Worthing Herald (Worthing, Sussex, England) of Friday 10th July 1942:
This is a war flying story of an altogether different kind, and presents the adventures of the bomber crew after they have “brolly-hopped” on to occupied territory and are being guided through Holland by the elderly rear gunner who is also a knight and an M.P.