ha (interj.)

natural expression of surprise, distress, etc.; early 14c., found in most European languages (including Latin and Old French) but not in Old English (which did, however, have ha-ha).

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Definitions of ha from WordNet

ha (n.)
(astronomy) the angular distance of a celestial point measured westward along the celestial equator from the zenith crossing; the right ascension for an observer at a particular location and time of day;
Synonyms: hour angle