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1.
Like a prom, but not held by a school. A group of friends holding their own dance (formal or not) complete with decorations and a hall.
Kira couldn't make it to prom so Josh held a Promish for her, complete with decorations and all their best friends.
by shawn82 June 06, 2007
 
2.
when one does NOT, in fact, 'promise'

*the suffix 'ish' can be used with any other word/s ending with the'iss' sound -to easily switch to the opposite meaning, can also MODERATLY change the meaning to a toned down version of the word :
do you prom-ISE....or..do you promISH?...
by mtroute March 06, 2007