While 'heddu''s exact origin and meaning are not know, one can safely come to the conclusion that it is a phrase used exclusively (and possibly created) by students of Mercer Island High School in Mercer Island, Washington. 'Heddu' is used, in the broadest terms, to express strong emotions such as joy, disappointment, lust, and surprise. It can also be utilized as a greeting, interjection, or 'filler' (similar to 'like,' 'um,' 'well').
Often pronounced with the second syllable stressed and elongated, depending on usage in specific context.
MI girl 1: Heddu, how was your weekend?
MI girl 2: Cool, cool...oh hedduuuu I went to this crazy party in Bellevue.
Mi girl 1: Oh nice, how was it?
MI girl 2: Good, I met this really hot guy and it was just like, 'hedduuuuuuuuuuuuuuuuuuu-uuu.'
MI girl 1: Aw, heddu. I wish I could have been there.