1. corruption of a word "atheist", probably first used in a information sharing website known as Yahoo! Answer (citation needed). the origin of the word is murky at best, but it probably derived from some n00b creationtits accidentally misspelling "atheist", thus creating failmeme in process.
it is now used here and there, exclusively on R&S board, as a jeer or insult at atheists.
this word is one of those cases where it can be used as both plural and singular, except if you know someone that has only one tit, then in that case call him or her athetit.
OMG, those athetits always rant about religion here on R&S!