The ugly fat girl (ie Land Monster) that is inevitablybest friends with a group of attractive females, or sometimes, the attractive female you are trying to have intercourse with. Like an NFL lineman, she will do anything she can to prevent you from attaining your goal. More often than not, the only way to get past this seemingly unpenetrable defense is to have a friend (prefereably a land monster slayer), hook up with the friendasaur. Much like a soldier in combat, one loyal slayer must jump on this perverbial grenade for the benefit of the whole battalion.
Guy #1: I really want to take Lauren home tonight, but you gotta help me outwith that friendasaur next to her.
Guy #2: Buy me 3 more whiskey shots and I'll slay her good.
def. An especially unattractive member of an otherwise good-looking group of socializing females.
Addendum: Also sometimes known as the "Fat Friend," one will observe that the Friendasaurus is sober and apparently PMSing, which aids in her dual roles of the outing; being the DD for the group, and as the GrandmasterCockblocker, foiling any attempts for an eventual sexual encounter with one of the more attractive ladies of the group.
Guy1: "Dude, 2 O'clock, the redhead, she's STUPID hot!"
Guy2: "Yeah, but you see the nasty chick dancing next to her? She's a major Friendasaurus; I just saw her cockblock that guy in the green shirt."
When an attractive girl has a surprisingly unattractive friend that always hangs around providing a constant living cock block for all men trying to make a play.