Phonetic abbreviation for the word facetious.
Typically coupled with the word "no" (i.e. "no facesh") as another way of saying "seriously" or "for serious".
Also pronounced and spelled "fasheesh" or "faceesh".
Another way of saying "seriously":
"Man, that dude was hella rude, NO FACESH."
To be half serious, leaning on mostly serious:
"You smell really really gross right now. You should put deodorant on. Obnoxious laughter. FACEESH! But really though."
When a joke isn't funny:
"That's so funny. FASHEESH."