Tucking the thumb into the palm or holding it in a way that emphasizes the length of the fingers or the daintiness of the hand.
The phrase is a slang term primarily used within the transgender community and online subcultures to describe a specific hand gesture or a visual characteristic sometimes associated with trans women and transfeminine individuals. t girl thumb
Much of the usage is "in-group" humor. Trans women use these terms to build community and share common experiences regarding transition and presentation. Tucking the thumb into the palm or holding