A corruption of the word Haole, Hawaiian slang for foreigner. Usually applied only to Caucasian people. Not necessarily derogatory, depends on context.
If you look up Haole on urban dictionary and spell it Howlie, you are probably a Haole.
Term used by locals in the state of Hawaii (usually not with a scrap of Hawaiian ancestry, usually Fillipino or darker-skinned) to refer to white people who moved to the state/island from the mainland.
It may or may not be meant as an insult. Depends on how it's used, by who, and to who.