The roots of this word are uncertain; some claim that it is a deformation of "difficulty" (which would be quite grievous for a typographical error) while others maintain that it is a derivation of "fugue." (However, "defugue" then becomes a nonsensical abstraction, as TheDiscouragingWord.com has pointed out.)
Since this word is not located in modern dictionaries, it is often mis-spelled. (See Defugilty.) However, the spelling found in all the internet references available (most of which are located on university webservers) is "defugalty."