Originates from the word "hope". In poker lingo, a player being referred to in such a way usually has loose and possibly passive tendencies. Typically used to describe players chasing gutshots and overcards.
Also applicable to a lot of people in everyday life. Real life hopeatrons are either irredeemably optimistic beings or simply lack the ability to pass proper judgment onto themselves.
“Hurpdy durp, my ace-high is nuts… I call!” – “What a hopeatron…”
“We’ve been upgraded to half-way decent? Before we were all like total hopeatrons…” Phil Laak to Mike Matusow in High Stakes Poker Season 2 Epidsode 1