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1.
A rehash is commonly used to refer to a remix of a song by another artist without much or any really change to the song itself. Seen a lot in pop, rap, techno music today, and very frequently with jazz and classical music in the earlier years.
Usually easily identifiable by the very similar, or exact-sounding lead, bass, or drum lines heard in a song.

Though, take care to remember that some DJ's and artists can very easily remix a song, that uses a lot of the same lyrics and lines as the original. But still add new beats, breaks, and a new feel to the song, and have the song be a legitimate remix.
Pitbull rapped over Nicola Fasano Vs Pat Rich's '75 Brazil Street', and made a rehash of the song.

'Oh man! I saw a band do a terrible cover of Led Zeppelin last night!'
'Really dude? I heard a DJ spin a terrible Rehash of Sandstorm last night!'
by Ricebowl Lee January 17, 2010
 
2.
to be an uber IRC pimp and to <3 mendy.
You're so rehashin these biatchez!
by rehash February 21, 2005