Guys have mathematical rules on the ages of socially acceptable sexual partners (half your age plus seven is a good one).
To do fuck maths is to mathematically work out whether somebody falls within the age limits.
I'm 20 she's 16, which is not quite half my age plus seven. But I did the fuckmaths and in 2 years I'll be 22 she'll be 18 which will make it OK.
Hey man if you dont take this new job serious you'll fuck it up!.Or Hey bro'congrats on your new job or engagement lets go out tonight and fuck it up.(party hard,Look for a fight. meet chicks.Get drunk etc..