The word 'menfess', if I am not mistaken, coined in RP (role playing) world. It is a combination of two words: mention and confess.

So basically it means mentioning someone (on social media) then confess something to them and all of this done anonymously.

'menfess' is not always a confession, it can be a message or even a question.

If you see “auto-menfess” then it provide some kind of service to post your menfess anonymously on a certain platform (usually a Twitter account, called base).

Hope this helps