Mentally-challended (a.k.a. retarded) kids use this word as an insult. Also used by mom's when they are tired. Possibly an abriviated version of "Dip-Shit"
Will: Dude check out that hot chick..
RT: Don't worry, you go get her - I'll stay with her ugly and anoying friend since that's all I can do!
Will: wingmaaaaaaaaaaaan *que coors light commercial*