Source: Public Service TV spot from Guinness featuring the animated Brewmasters.
1) Acronym for the message of above-referenced spot: Use Common Sense And Always Know When You've Had Your Fill
2) General sign-off to your buds as they head out for the pub/club/party
3) Expression of disbelief, akin to WTF
2) Guy 1: "I'm heading out for the club to get laid, drunk or both.
Guy 2: "Good luck, Dawg. UCSAAKWYHYF."
3) Guy 1: "Dude... when I was out, I chatted up that 'girl' you were drunk-dancing with last week. 'She' asked for your number - and said thinking about your pants gave 'her' a boner!"
Guy 2: "UCSAACWHYHF!"