This is where an account puts a picture of themselves up for a couple of minutes. It is usually a nude or a very revealing picture and the poster sometimes pretends like they didn't mean to upload it. (see: bullshit) it is usually done as a desperate attempt to gain more followers and attention.
"@imamassiveslut: quick #picslip of my body for you guys ;-);-);-) remember to follow me!!!1! xoxoxoxo'