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".