Slang for a penis because as one cums, (unless you're really fucking old or have had some sort of injury) the first few orgasms squirt.
Johnny McWarguy: Hey Jen, wanna see my squirt gun?
Jen: Uhhhm... ok?
Johnny McWarguy: *unzips pants and squirts his load in Jen's face*
Jen: ...I'm going to kill you.