The Pipebomb is a hand-held explosive. A combination of the code, the only screenshot it appeared in and it's HUD suggests that it would be an explosive weapon that could be remote detonated. The player would toss the bomb, then could detonate it at anytime. They could also pick up already-thrown Pipebombs for whatever reason, as long as it wasn't blown up.
The Pipebomb was removed from the final game, but it's HUD icon can still be found in the final game's files and the Serious Sam 1 SDK. Why it was cut is unknown. The Pipebomb's behavior did appear in the C-4 Demolition Charge found in Serious Sam 3: BFE.