When someone thinks they made a good comment, joke, statement, or accomplished something when in reality it made them look like a complete idiot.
Steve: Your mom was good last night! Oh I got you soo good!
John: Congratufuckinglations my mom is 76 and overweight. Good one man.
Or, Max: Yes I finally beat level 25 im fucking awesome.
James: Wow, congratufuckinglations why don't you go outside now you geek.
A general state of feeling like the boss. It is not referring to being at work. It is more of a state of power, a temporary feeling. You are the shit for a minute. Used as an adjective to describe a feeling.
You must feel so foss with that new car man.
"That outfit your wearing makes you look like the boss."-Person 1
"Yeah man, im feeling foss."-Person 2