On of the sacred words kept by the knights who say "NI".
It can also be used as an expression of contempt.
Extending the "e" sound or chanting the word in poorly synchronized chorus can express further contempt.
Me: Hey, lets go get some ice cream.
Friend 1: Yeah, Nee-wom!
(Friends 2, 3 & 4 simultaneously)
Friend 2: Neeeeee-wom!
Friend 3: Neeeeeeee-wom!
Friend 4: Nee-wom!