To send unsolicited pictures of your genitalia to another person online, typically with the hope of reciprocation.
"This random dudebro just penced me on Facebook Messenger. I replied with a gif of a banana being sliced and he blocked me."
Short for "baseline", a sline is someone with no marketable skills or useful knowledge nor hope of acquiring same. Essentially, a normie that has no hope of improving.
I was at Starbucks the other day and a bunch of slines came in, demanding Unicorn Frappes.
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