Well it is often used as slang for transmission. For example the lovely
Marisa Tomei as Mona Lisa Vito in My Cousin Vinny used the term "tranny" to
describe the transmission in a metallic mint green 1963 Pontiac Tempest,
with a white convertible top, Michelin Model XGV tires, size 75-R-14.
Also slang for transistor radio, transsexual, transvestite.