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1.
Having sex. When one checks in or is checking in, it is either to the Vagina Hotel, or the motel (the butt).
Guy 1 "Hey buddy, were you checking in last night?"
Guy 2 "Nah man the hotel had no vacancy, but I'm gonna try again tonight"
by eyememine January 03, 2010
 
2.
Arriving at a destination to purchase some goods for a 'mate'.
'I've just been Checking In for Brian at the local shop.'
by Bill Carr February 12, 2007