Brewer's: Jump Over the Broomstick
(To). To marry in an informal way. A “brom” is the bit of a bridle; to “jump the brom” is to skip over the marriage restraint, and “broomstick” is a mere corruption.
“A Romish wedding is surely better than jumping over broomstick.” —G. A. Sala.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894