A moment that has happened sometime in the past (usually the night before) and is tremendously embarrassing to even think about never mind speak about. When a 'No' moment has been recalled usually the victim will come out with a frustrated
"NOOOOOO!" groan or even a delicate "no..." mumble. 'No' moments are usually caused by alcohol and cringe-worthy social situations.
"Aww man you were totally wasted last night and you tried to pull Jessy! But she refused your sexual advances! Gutted for you!"