a text sent by a girl (usually to another girl) full of fakeness. Being fake includes pretending to like her when you really just want to punch her in the face, complimenting her when really you're just making fun of her, and/or saying you need to hang out when really you'd rather stab yourself in the eye.
Another other forms of being fake are also considered fexting when they are sent via text.
Lisa: "I hate that bitch but I need my stuff back. I'm fexting her."
Fext sent: "hey girl it's been way too long! We need a date soon! I miss your face :)"
Real text meaning: "I hate your fucken face but I want what is mine so hurry up and fext me back so we can pretend to be friends one last time."
Lisa after fexting: "We were just fexting and we're meeting for drinks tomorrow. God I hate that fucken bitch."