Tranny can mean one of two things. In most cases, tranny is used as a derogatory slur for a transsexual person. In the automotive sense, tranny is a term for transmission.
"There goes another one of thosetrannies."
"I'd rather replace a torque converter or a clutch rather than an entire tranny."
An extremely offensive slur that targets transgender people. As offensive as the N-word and the f-slur. Don't even say this word if you aren't trans (i am)
If you hear someone say this word or is being transphobic i strongly suggest you kick them in the nuts.