This is such a thing of impeccable beauty that I had to repost it! 
NicolaSturgeonsSOGIbottom
I like a simple ‘So what? Your mum’s a terf’
Because 1) classic ‘your mum’ joke, but with the usual misogyny reversed (you are putting yourself and the mum on the same level, not denigrating her, whilst implying that the ‘found the terf’ person is likely a sexist dick towards zir’s mum on a regular basis.
2) Terves are cool because they speak up, don’t put up with stupid shit and call a spade a spade (well, a man a man, but you know what I mean!)
3) wite it in little letters, it makes it look like the silly made up word it is m, diminishes their power and demonstrates that they use at as pejorative noun for women who don’t pander to men, not as neutral factual acronym for a particular strain of thought amongst radical feminists.
4) it’s probably true. Even the crazy rainbow munchiemoms know deep down that their sons will never be women in the exact same way they are. It’s why they treat them like child prodigies or the strange child saints of history. Ordinary female daughters wouldn’t get a fraction of that raptured attention. Mums of boys who transition as young adults are generally really perplexed by it all, but might try to act supportive because they’ve been conditioned into with all the suicide baiting. There are loads and loads of posts on trans subreddits moaning about how their mum/dad/granny said something that ‘seems a bit terfy’ and it’s usually something really innocuous like asking a 6ft plus adult son to take the bins out because they are stronger than their middle aged mum.
5) It’ll make them paranoid.
Finally:
The only way they can defend against it is by listing all the trans ally stuff their mum has done for them, which will look utterly stupid as a response to a ‘Your mum’ joke.
They can’t even report you for calling their mum a bad name, because they used it first, so they’ll have to admit it’s a pejorative.
If they try a pathetic ‘No she isn’t’ just say ‘yes she is, I know her from Mumsnet’
If they say ‘My mum is dead’ just say, ‘Ah, sorry, thanks for the correction. I shouldn’t have said ‘Your mum IS a terf’ I should’ve said ‘Your mum WAS a terf. RIP Tervern Sister’
and then some obnoxious flower and heart emojis.
This ideology cannot cope with being laughed at. It needs utmost seriousness to survive.