Baest is an alternate spelling of "beast" that originated from Modern Warefare Clan Tags. The Clan Tags could only hold four characters and to spell the commonly used slang "beast," one had to use the Latin æ sigma to spell the word in under four characters. The Clan Tag gained immense popularity and began to be used in common internet usage, being spelt "baest." The definition is still essentially the same, other than it has the reference to Modern Warefare.
DUde! EMINƎM is so BæsT!
(Joe) That was such a baest porno! I had an epic boner.
(Mike) Dude, that was Hannah Montana, you perv.