An exclamation of surprise, excitement, and/or pleasure. It was possibly coined, and certainly popularized, by the character of Zonker Harris in the Doonesbury comic strip. Zonker was delightfully weird, and so is Heewack!. Although heewack has been around since the 1970s, it has fairly steadily failed to gain popularity in any context, and thus its use is a strong indicator that the heewackee is delightfully weird.