To block, reject, or ban from something. Often used as an insult to the n00b being admined.
John: Oh look at this smacktard, he's spaming the forums!
William: You better do something about it quick before he hurts himself.
John:...Yeah i just admined his ass.
To make somebody an admin.
especially to admin someone on a facebook group.
Dude, I just admined you on my facebook group.
Yo whore who the fuck admined you to THAT?