What does this mean?

i think it refers to a totally inept person...

I think the meaning is of a drunk suddenly changing position from vertical to horizontal.

This site is a wonderful place to fritter time.

http://www.manwalksintoapub.com/words.htm

"Couldn't hit the floor with his hat" sounds like a person who can't hit a target -- "can't hit the broad side of a barn" is another such idiom.

I don't know what it means without the negative...