to have a large bum or behind.
"that new kid louis has a rather 'BIGBATTY'!"
Big batty bum bum ---> someone who has a large BATTY
"look over there, its louis, he has a BIG BATTY BUM BUM!"
bigbattybumbum means bigbum in uk slang
Look at Louis over there, he has a bigbattybumbum!