The squinty face that one suddenly and uncontrollably makes when the DJ drops that sick-ass funky track.
The faces the entire crowd makes, in unison, upon hearing that catchy, funky hook in a dance song. Sickfaces are most likely seen where House music is being played.
Oh, snap! That track was off the hook. I had my sickface on the whole time...
Holy shit, did you hear that track?! Everyone had their sickfaces on man!