ha (interj.)

also haha, used of laughter since ancient times; Old English ha ha. Also in Greek (ha ha, in Euripides, Aristophanes), Latin (hahae). A different attempt at representation in English is py-hy (1580s). Sometimes interchanged with ah and expressing surprise, distress, etc. A ha-ha (1712), from French, was "an obstacle interrupting one's way sharply and disagreeably;" so called because it "surprizes ... and makes one cry Ah! Ah!" [Alexander Le Blond, "The Theory and Practice of Gardening," 1712].

also aha, exclamation of surprise or delighted discovery, late 14c., from ah + ha.

This seely widewe and hire doughtres two ... cryden out "harrow!" and "weloway! A ha! þe fox!" and after him they ran [Chaucer]

variant of ha.