(In Laymans terms, you say you're just going to have some drinks at the bar/club/party, but really you intend to hook up.)
Or, if your single, then you say it when you're planning to go to a bar/club/party, drink alcoholic beverages, and get laid.
Girlfriend: Wait - I can get ready real quick and come with you!!
Boyfriend: Nah baby, there's no need for you to get all prettied up, I'll only be a few minutes.... I swear!
Boyfriend goes to party.....
John: So where's your girlfriend.
Boyfriend: Man, I can't have her with me when I'm cocktailing... what are you thinkin??
John: Damn straight!
Boyfriend hooks up with hot chic at party and hurries home, girlfriend none the wiser.
IF YOUR SINGLE:
Friend 1: "Hey, lets go out cocktailing tonight!"
Other Friends: "I'm game."