Short for "Not Mind Safe" (and not for "Not My Style", what kind of lame "street" phrase is that anyway). Indicates content that so disturbing it should be forever left unseen. Used as an extreme warning for images whose full horror is not covered by NWS (Not Work Safe).
Wow, I puked all over my keyboard after seeing mrhands.avi (warning: NMS). That was one well-hung horse.