Well, who DID you steal it from??
From
http://www.worldwidewords.org/qa/qa-bum2.htm
[Q] From David Sinclair: “I recently heard an American use the word bumbershoot as a humorous term for umbrella. I cannot find where and when it originated. My dictionary says it is an Americanism, but some web sites have said it was a British word for umbrella. The chute part suggests it is recent, but it frequently is associated with old folks, especially ones in the countryside. Any help?”
[A] Any suggestion of a British origin can be immediately refuted. It isn’t known over here at all. In fact, I’d never heard of it until you asked your question. It appears in the lyric of a song sung by Dick Van **** in the film Chitty Chitty Bang Bang:
Me ol’ bam-boo, me ol’ bam-boo
You'd better never bother with me ol’ bam-boo
You can have me hat or me bumbershoot
But you'd better never bother with me ol’ bam-boo.
The English context of the film may be why some Americans, not familiar with the word in their own country, have come to believe it must be British, though the song was actually written by two Americans, Richard M Sherman and Robert B Sherman.
The first example of bumbershoot in Professor Lighter’s Random House Historical Dictionary of the American Language is from 1896. There were some variations around in the early days, such as bumbersol (with sol presumably taken from parasol) and bumberell. By the first decade of the twentieth century it had settled down to bumbershoot.