Omorashi in Japanese means “to wet oneself”. A paraphilia involving urinating on oneself but also arousal brought on by having a full bladder, causing someone to develop a full bladder, or by witnessing someone with a full bladder “relieve” themselves.
Often also abbreviated as OMO. It was so shocking for me to realize how huge and how many fans this has in Japan. That’s why in JAV this gimmick returns all the time.