In Omaha Hi-Lo and other lowball poker games, the "nut low" refers to the absolute best possible low hand. In Ace-to-Five Lowball, the nut low is A-2-3-4-5, while in Deuce-to-Seven Lowball, it's 7-5-4-3-2.
The term is often used in regular high card poker games to refer to worthless hands, which is the way I’m referring to Rupar.