Possin can be used as both a compliment and an insult, depending on context. It's mainly used to call someone attractive, but it's also used as an insult, and basically the adjective version of pissum.
"Oh, wow! She's looking possin in that dress!"
"Ugh, I hate him. He's so possin."
Related to the adjective "possin." Pissum is an insult, used on pretty much anyone, but its meaning is 'whores that piss on the streets for fun.'
"Joey is literally a pissum. Do you see what he does at night?"
"Damn all these pissums at this party!"
"Winry, I've gotta divorce you. You're just such a pissum."