The male or female form of interaction, with the opposite sex, including flirting or attempting to pull.
Or more loosely defined as any interaction/banter/chilling/pulling between any guy or girl.
Jem: Mate are you schweffing tonight?
Harry: Yeah thinking of schweffing the floor 8 girls actually, fancy schweffing along?
Jem: Yeah I was gonna see if Steve was up for a schweff, but apparently he's schweffing Ellie!
Harry: OK.. Schweff ya later!