Covertly ntering your partner from behind while under the guise of doing it doggy-style. As soon as he.she relaxes, screaming "Look out-Big Bird!" and vomiting on his/her head. Also can be used to describe having anal sex with multiple partners in a train fashion when vomiting on the person in front of you when one person yells "Look out! Big Bird".
Kevin had a bunch of circus midgets over at his trailer last night and they all Machaned - it was a mess.