I came across a comment:
What is the meaning of 怎么怼回去?
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怼 is fairly new internet slang.
MDBG defines it as:
(Internet slang) to attack verbally / to publicly criticize / to call out
notice: it is read in the third tone here and basically means to lay into.
With the rise of popularity of 怼, many new words came out and have started popping up everywhere.
Not to get off topic too much, but another common new word is 怒怼.
(Internet slang) to chastise / to angrily denounce
This hopefully gives a better overall view of what 怼 actually means.
Another new word, that would answer your question more clearly, that has come out is 回怼:
to retaliate verbally / to hit back (at critics)
So back to your question:
Brazilian Jiu-Jitsu has been attacked and now they’re asking how to retaliate or how to hit back at their attackers.
duǐ, in fact, should be written as 㨃. It's originated from the northeast dialect. It's been written as 怼 on internet now for some reason. I guess it might be because we don't know the correct writting originally and just find one from any Pinyin Input.
The idea of 㨃 is that: to push, to knock against, and figuratively to talk against, or fight back, etc.
怼= 懟 as in 怨懟 (resent; resentment )
怼是一个汉语字，拼音为duì，基本意思是会意字，说文，心部，从对声。对，相持也，意为互相对峙，底下加“心”，表示心里抵触，对抗。引申为怨恨，故而怼，怨也。(therefore 怼 means 'resent')
'对' in '怼' means '对抗' (to confront)
'心' in '怼' indicates 'from the heart; emotionally; mentally'
'怼' means 'resent'
'怼回去' seems to mean 'respond with bitter confrontation'