If you actual read my comment correctly you would see that I said bootleg used to mean mashup, some people still refer to mashups as bootleg.

For a long time, in popular music, a bootleg has been an unauthorised copy of some music. That would include unauthorised concert recordings, copies of masters stolen from record company archives, or just cassettes dubbed from commercial LPs. At various points in history, you'd find bootleg recordings as vinyl pressings, acetate pressings for DJs, as professional looking counterfeit cassettes, as obviously home-dubbed cassettes swapped by fans, or sold at markets. Later, of course, cassettes made way for recordable CDs.

As Wheat says, the term comes from smuggling and dealing in prohibited goods (for example, liquor was traded by bootleggers during America's prohibition years).

In around 2001, a scene emerged that became known as the "bootleg scene", or the "boot scene" -- in which people combined two well known records to produce a pleasing combination. For example Freelance Hellraiser combined an instrumental of The Strokes' Hard To Explain and the vocal from Christina Aguillera's Genie In A Bottle, to produce A Stroke of Genius.

These "boots" were distributed as MP3s on the Internet, and also pressed as limited release singles. Since they didn't have permission from any of the original copyright holders, these were bootlegs in the truest sense.

However, it's important to understand that an unauthorised recording of a Rolling Stones concert is also a bootleg in the truest sense.

When the record companies realised the popularity and commercial potential of the early 2000s mix-and-matching-records scene, some of them went on to produce authorised music in the same vein. For example, Richard X's combination of Adina Howerd's Freak Like Me and Gary Numan's Are "Friends" Electric was released as a single, with the vocals re-recorded by the Sugarbabes, and became a big hit. 2 Many DJs's "As Heard on Radio Soulwax Pt. II" -- a continuous mix -- was audited for copyright use, and released officially with great success.

Once everything was authorised and copyright-cleared, it was no longer appropriate to refer to these as bootlegs, so somebody coined the word mashup. Two (or more) recordings are mashed up together.

It's also important to note that this doesn't have to be an electronic process. While most creators of mashups do use computer software to align samples to each other, some do it all with vinyl source material, and turntable skills.