When the sex is so unbelievably awesome that after orgasm you immediately forget what you were talking or thinking about.
I was talking dirty to my wife while we were having sex. She got so excited, clamped down tight and gave me the best orgasm ever. She had to repeat what I had told her since I couldn't remember. Instant case of sexnesia.
by nrs1970 June 15, 2010
Having sex with someone so horrible you force you mind to block out all memories of the entire preceding 24 hours
Dude...what happened on Thursday, that chick was so ugly I have sexnesia...
by betsyrees910 May 08, 2008