It once was a combination of the words 'For' and 'Sure'. And while the word can still mean that, it can also apply to anything the speaker finds cool or sick or dope.
Yo Alex, I found the most fosh rock formation the other day, it was shaped like an eagle with a dragon in it's talons
Past tense of fish.
I found a fish, I foshed it and then I left.
"FOSH" is a versatile slang term primarily used to call out falsehoods or absurd statements, often indicating disbelief or skepticism. For instance, if a friend claims they won the lottery, but you know they're exaggerating, you say, "FOSH" Similarly, if you spot someone whose ugly, you say, "That bitch is foshed," expressing disdain or disapproval.
Bro the girls at the party are FOSH asf or FOSHED
Max Fosh Is An Absolute Mad Lad
Max Fosh Is A Really Cool Youtuber
means Most Definitely, For Sure
Most Definitely is broken down into M.D. which is the abbreviation for medical Doctor, which then is shortened to "Doc"
Then For Sure is shortened to "Fosh"
"do you want to go to the club tonight?"
"Doc Fosh"