On August 23rd, 2016, the term "Egg mode" was defined by Urban Dictionary user bitplane7 as "Someone who is trans but doesn't know it yet. Someone who is still an embryo of the trans person they will eventually hatch into." The definition gained 106 upvotes and four downvotes in three years. On August 9th 2017, Urban Dictionary user happycookie defined "egg" as "Transgender person who hasn't come out yet. A chick or a cock on the inside." the definition has received 329 upvoted and 89 downvotes in two years. It has since become the top definition for this term.

-Know Your Meme

I always thought the origin was going to be some dumb joke like "crack an egg and a chick comes out", somewhat surprised to see I'm right.