When you're doing a girl (or guy) from behind, before you cum, spit at her (or his) back so she (or he) will think that you have ejaculated, and will turn around. When she (or he) turns around, ejaculate on her eye, making her (or him) scream "arrrrgggh" hence the name pirate.
Dude, James just pirated his boyfriend
last night. His boyfriend
's now wearing an eyepatch