couldn't stop a pig in a passage

a short drop and a sudden stop

A fall to one's death by hanging.

couldn't be more different

Two entities that have virtually nothing in common.

couldn't care less

To not care at all; to have no interest or concern; to be apathetic.

couldn't carry a note in a bucket

Not musically talented, unable to sing.

couldn't happen to a nicer

Sarcastically asserts that those in question thoroughly deserve their fate.

couldn't organise a piss-up in a brewery

Is unable even to organise a project that requires no planning; is incompetent; used to describe someone as greatly incompetent.

face that would stop a clock

A shockingly unattractive face.

happy as a pig in mud

Very happy or content.