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1.
Any weapon left to you in a will by a relative. Not necessarily a pistol, or even a service weapon, but usually. Generally left with an instruction not to use it for anything retarded. Usually in very good condition.

So named because of the stereotype of a grandfatherleaving his favorite grandson his service pistol.

abbr: GSP
Guy1: Hey, where are you going.

Guy2: Taking Grandpa's Service Pistol to the range...

Guy2: But... that's a shotgun.

Guy1: Yeah, "Grandpa's Servoce Pistol" is just what I call any weaon left to me by a dead family member.
by Al Golston November 20, 2011