Not Quite Right In the Head

The world is generally more interesting with people who are "different".   At Thanksgiving dinner last night I heard a saying that seems to be common in America, but due to my foreign background,  I never heard before - the saying is "Half a bubble off level"  or "Half a bubble off plumb".    It's sounds pretty American in origin.  It seems that the term was used by none other than  Samuel Langhorne Clemens (aka Mark Twain).  This seems about right given that Mark Twain started out as a typesetter and piloted a steamboat on the Mississippi River.  However,  it is very possible that Twain picked up the saying after he moved east to New York and Connecticut.   Twain  loved science and technology  - he was a friend of Nikola Tesla and they used to hang out together in Tesla's lab. Twain  lost a lot of money financing new inventions. 

Now we get to the issue of the meaning of the saying.   I guess the pithy term would be eccentric. But eccentric has several meanings  -  some quite unrelated -    off-center or outside,   imbecility,  folly , insanity,  capriciousness, un-conformity or non-conformity, ridiculous, comical, odd...wow, ain't English a fine language for insulting people.   :)