To
copy,
distribute and generally
profit from something
illicitly; that is, without the owner of the (intellectual)
property also getting due
compensation.
In more broad terms, to distribute something in contravention of a law or bylaw against it. Bootleggers during Prohibition snuck booze into the US - bootlegging is nicer than calling them smugglers, and generally carries a more lenient punishment. Pirates bootleg software and traffickers bootleg narcotics.