greeting, 1862, American English (first recorded reference is to speech of a Kansas Indian), originally to attract attention (15c.), probably a variant of Middle English hy, hey (late 15c.) also an exclamation to call attention. Extended form hiya attested from 1940.
A salutation upon meeting: The staccato cry of ''Hi!,'' which we judged to be the almost universal greeting
[first form 1862+, second 1940+; probably based on hi, ''hey,'' exclamation used to call attention, found by 1475]