Theyster Share definition Flag A Theyster is a non-binary friend who uses the they/them pronouns. Instead of using brother or sister, the word is fused into THEYster. My non-binary friend Pat is a good Theyster. They always help me out when I need a hand! Theyster by PseudoNympho666 September 1, 2023 0 0 Get merch Get the Theyster mug. Buy Now — $32.95 See more merch