acronym stands for Single White Female. is also used to refer to a girl who develops an obsession with another girl (whether out of jealousy or crush) and begins to act like her, either knowingly or unknowingly. sometimes referred to as stalking, but more of a thin line between love and hate.
can be used as a verb as well.
"wow, she found your livejournal and started quoting it as her own words in her blog? she totally SWF'ed you!"