A scabby anus (ie. An anus and/or surrounding area that has been scabbed over.) It is most commonly recieved from being rubbed on the sidewalk, though a scaj can also be obtained from an STD or a rough night.
When used in referance to one's self, a scaj is just a synonym for butthole.
"Timmie just stuck a beer bottle up his butt, he's totally going to get a scaj."
"...Mom, they felt me up, they tried to look in my scaj."