It is accurately defined as a phrase used in an intense situation in which something good or bad has happened and "aw snap" or "snap" are not effective.

Marginalized Orientations, Gender identities, And Intersex. it’s meant to be an all inclusive umbrella term for asexuals, homosexuals, multisexuals, trans people, and intersex people. Alternative to LGBTQIAP+
Alternative to MOGII, easier to pronounce.

Fasnaps is a type of fake Snapchat account. Kind of like a Finsta (Finsta is a fake insta for funny and embarrassing pics aside from a normal Instagram) so a Fasnaps is a fake snapchat account for close friends to see you posting what you normally are like.