Meaning of “Yuri(百合)”

“Yuri(百合)” means lesbianism as a slang.
Originally meaning of it is lily.
I don’t know why the meaning of it became lesbianism as a slang…

Example Conversation

A: Is it true, this season’s Gundam is yuri(百合)?
B: I thought, but it might be OtomeGa(乙女ゲー)…

A: 今期のガンダム、百合ものってマジ？
B: 百合だと思ってたんだけど、乙女ゲーなのかもしれない…

*Gundam is a really popular robot anime.