n.) Someone who is an obvious bullshit liar. v.) The act of making a bullshit lie.
Of course this can also be used as an adj, adv, etc. The main idea here is that the word "fossitt" somehow relates to the most blatent, obvious lie you have ever heard.
n.) "Hey Tim! I won the lottery!""Alex, you're such a fossitt". v.) "Hey Alex, my car can go 300 MPH!""Really?!""No, I'm just fossitting ya! lol".
A place where the FOS (full of shit) go to be cured of compulsive lying through a combination of electro-shock therapy and regular slaps in the head. If there is no fospital in your vicinity, please apply first aid (Swift slap in the head)