The act of a man (or a properly equipped woman) inserting his penis/her pseudo-penis into a womans anus (the two hole), then into her vagina (the one hole) and finaly back to the anus (two again) in quick succession, or 2-1-2, a Manhattan area code.
Bob: Soooo, what happened with Jenny last night?
Frank: I gave her a Manhattan
Bob: Oooh, so can she hold her liquor?
Frank: I wouldn't know, she's been in AA for 3 years