Very slickindeed.
Phrase used to convey:
1)A sense of elegant suavity
2)A smooth, dashing je ne sais quoi
3)Class coming out ones' ass
Impressed Socialite #1:"I say, did you see how Williambutt-fucked that bunny-rabbit without soiling his cummerbund or spilling his green tea?
Impressed Socialite #2:"Yes, he's as slick as snot on a doorknob."
Kevin: bro I got my dick stuck in a doorknob yesterday
Brian: why the fuck was your dick in the doorknob in the first place
Kevin: because bro, it's national fuck a doorknob day