A sunburnt foot is the area on the back of people that is inflexible and reached with great difficulty when one wishes to apply after-solar exposure cream. In elderly people it is often known as the sunburnt yard.
dude1: 'Heeeeey, you know what they say about people with a big sunburnt foot?'
dude2: 'Hell yeah..."
A sunburnt arse is a person whose face has become so swollen with sunburn that it looks like an arse (of generic appearance). Typically of English origin. There are no know cures.
babe: 'hey whats that baboon on heat doing staring at my tits ?'
dude: 'thats just a sunburnt arse!'