What is the origin of "willies" as in "that gives me the willies"?

No one knows for sure where this word originated, a state which has invited broad speculation. The Morris Dictionary of Word and Phrase Origins claims that it is a reduction of willie-boy "sissy", but that hardly makes semantic sense. William Morris, the Word Detective, opines that it comes from the name of a Slavic sprite called a vila (vili "sprites") sometimes translated as wili.

However, spelling the name of the vili with a W is German, where the letter W is pronounced [v]. The best guess in my opinion was suggested by Jackie Strauss, who also suggested the word itself. This word reminds Jackie of the woolies, which is to say scratchy wool long winter underwear. The willies are the same as the creeps, which suggests an uncomfortable skin sensation to me, too.

http://www.alphadictionary.com/goodword/word/willies

The willies is basically the feeling of nervous apprehension. It is sort of like your own subconscious is telling you to be alert, be wary. An early warning system in case of danger. Sometimes our subconscious can pick up on minute details that our conscious selves aren't aware of and the willies is the way our psyche communicates with us.
The origin of the word is unknown but there are two possible theories of where it may have started. One is that it is a shortened version of the slang expression " willie-boy, meaning sissy. The second is that it may come from the Serbo-Croatian word "Vila" which translate to the English word "wili or willi", having the meaning of wood-nymph, fairy, or spirit. Possibly the ghost was referred to as the "willies"