imitative of laughter, Old English.
Ha ha and he he getacniað hlehter on leden and on englisc. [Ælfric, "Grammar," c. 1000]
mid-13c., from Old French ha, las (later French hélas), from ha "ah" + las "unfortunate," originally "tired, weary," from Latin lassus "weary" (from PIE root *lē- "to let go, slacken"). At first an expression of weariness rather than woe.