Green’s Dictionary of Slang

(US gay) a pre-operative transsexual.

1998		R. Scott Rebecca’s Dict. of Queer Sl. 🌐 pre-op — pre-operative transsexual, one who has not yet had any surgeries but intends to do so.
2017		T. Pluck Bad Boy Boogie [ebook] ‘Did her time pre-op’.