LITERALLY it means nothing, because mud is not clear. The word "literally" means to take the phrase exactly as written and not define it using slang or metaphor or anything else.
they have something that's called a vaccum and its a special type that's used for pools. i don't know exactly what its called but you will have to ask and find out.
It means if someone is giving you an explanation about something or you read something that is confusing 'it's clear as mud' meaning it's confusing and not clear in your mind.
Can't find an exact date as to when Flanders and Swann wrote it but they were active between 1956-1967. (I believe it was recorded in 1959) see the related link below for more information.