Adjective, used to describe something that is excellent, particularly apt when the excellence was unexpected.
"Walking home last night I bumped into these two Finnish sisters looking for a houseparty and ended up shagging them both in some guys spare room!"
"Fucking hell mate, buce!"
Also used as a subtle, quick method of agreement in situations where a longer and more enthusuastic response would be inappropriate or even dangerous. Such as letting your mate know you clocked that bird with the big knockers that just walked past when you're with your girlfriend.
A knowing look might be passed and a quick "buce!"