The process of shagging a male or female then tagging them in a social networking post in the morning after the deed is done, thereby making all social media aware that you have enjoyed sexual relations with that person. Pictures, videos and locations that can be added to the post make the shag tag more meaningful.
Have you seen the Shag-Tag on Facebook?
Steve Shag-Tagged Jenny!
I can't believe Sarah posted a Shag-Tag with Mark!
John is in bed with @Linda
dude1: Did you ever hit that julia girl?
dude2: I totally creamed on her shag-tag.
dude1: Nice. she did like it doggy style.