Luna Undies are period underwear, designed to function on their own as a menstrual product. In New Zealand men's underwear is known informally as "undies" or "dacks". In Australia, men's underwear also has various informal terms including "under-dacks", "undies", "dacks" or "jocks". Colloquially, in Australia, "Reg Grundies", "Grundies" or "Reggies", is rhyming slang for "undies" (underpants). Steffy has an interview with Liam in her pink and black trimmed undies. She Luxed my undies in darkest Africa. Favorite Moment&colon; Finally figuring how to change the color of Sophitia's undies.