"Hiya" doesn't mean anything in Japanese, but independently "hi" can me "fire" or "heat" and "ya" can mean "shop" or "store" or "eight" depending on what characters are used.
It is a grammatical shortening or grammaticalization of "How are you?" resulting from habitual use. Hiya (How you) served as an intermediate form. It developed in the course of the 18th century with hiya apearing in recorded text as early as 1940, though it likely was in use before then.