When you say you're going out to have some cocktails... but you really plan to get some tail.
(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.
Boyfriend: Hey baby, I'm going to John's party to say Hi and maybe have a quick drink... I'll be back shortly.
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."