A skinny margarita is a low-calorie version of a regular margarita. Instead of using an orange-liqueur like triple sec, Cointreau or Grand-Marnier, a skinny margarita uses fresh orange juice and a small amount of agave nectar.
Order “Skinny” margarita, only it doesn't have fake sugar (you'll have to ask your server/bartender). Ask for light to no agave/simple syrup. Tequila neat/straight up with a lime. Ask to swap in Triple Sec, Cointreau, or Grand Marnier in place of a margarita mix, sugar, or agave.