used most commonly by 'derro australians' and popularised by rodney rude; fuck ass is used to emphasise and describe something that is small. the phrase 'fuck ass' is usually used in association with the words small, little, tiny etc but can effectively be used on its own in a sentence.
"man what the fuck, i ordered a burger not a fuck ass little dinner roll"
person 1: dude that fall was pretty bad you alright?
person 2: yeah im fine, just got a fuck ass cut on my arm.