ce51f366_Local_Slang__Saying
[Explanation] Hassock over surplus means falling, stumbling, to trip over. The hassock being a kneeling mat/cushion in churches and a surplus is part of a clergymans dress - and in the saying you can imagine a vicars legs going in the air and his 'uniform' following. [Saying] 
'assock o'er suplas
Local Slang
Saying