1. Used to show something so incredibly immense-ified and huge-mongous, that a regular word, and indeed, two words stuck together (see above) cannot even begin to describe the subject in question.
2. Used also to describe something that is extreme, takes you aback, or over-the-top. The term is used because saying "Over-The-Top", or "OTT", makes you look like a fuck-faced loser, or an art teacher.
3. See also: "Fuck Off".
One would normally use this is a circumstance where just saying "big" would not do, and saying something like "huge-mongous" would make one look like a twat.
Origins: The Tiffin Swing Band/TATB tour to Poland, in particular, Callum and Noosh.
"Look at that fucoff pimp suit!" (See 2.)
"Fucoff, Cocksey, no-one likes you." (See 3.)