A friend once brought up an interesting point:
The way you know that “tranny” is a slur is by doing 3 different Google Image searches.
1.) Search “trans woman” and “transgender woman”
2.) Search “tranny”
Do you see the difference (I had to turn Google’s SafeSearch feature on for that 2nd search as the results were a bit on the raunchy side)? Do you see how when you call me (or any of my other trans friends) a “tranny,” what you’re saying I am? The top images are made up of confident trans women (yes, there are a few misguided entries, but hey, it’s Google, it’s not perfect). The bottom image is mostly comprised of bearded men in drag, photos that demean the people in them, jokes, pornography (take the SafeSearch off and you’ll see).
Next time someone claims “tranny” isn’t a slur, pull out your phone and do these quick searches, asking them if they can see the difference, if they can see why it might not be the most polite thing to call trans women.