Etymology
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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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a-ha (interj.)

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]
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ha-ha 
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].
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aha (interj.)

expression 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]
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Hanoi 
city in northern Vietnam, from Vietnamese Hà Nôi, literally "River Inside," from "river" + nôi "inside." So called in reference to its situation in a bend of the Red River. Known 18c. as Dong Kinh "Eastern Capital," which was corrupted by Europeans into Tonkin, Tonquin, and that name was used in the French colonial period to refer to the entire region and extended to the gulf to the east.
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he-he 

imitative of laughter, Old English.

Ha ha and he he getacniað hlehter on leden and on englisc. [Ælfric, "Grammar," c. 1000]
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alas (interj.)

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.

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hay (n.)

"grass mown," Old English heg (Anglian), hieg, hig (West Saxon) "grass cut or mown for fodder," from Proto-Germanic *haujam (source also of Old Norse hey, Old Frisian ha, Middle Dutch hoy, German Heu, Gothic hawi "hay"), literally "that which is cut," or "that which can be mowed" (from PIE *kau- "to hew, strike;" source also of Old English heawan "to cut;" see hew).

Slang phrase hit the hay (pre-1880) was originally "to sleep in a barn;" hay in the general figurative sense of "bedding" is from 1903; roll in the hay (n.) is from 1941.

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see-saw (n.)

also seesaw, 1630s, in see-saw-sacke a downe (like a Sawyer), words in a rhythmic jingle used by children and repetitive motion workers, probably imitative of the rhythmic back-and-forth motion of sawyers working a two-man saw over wood or stone (see saw (n.1). Ha ha.).

In reference to a game of going up and down on a balanced plank, it is recorded from 1704; figurative sense is from 1714. Applied from 1824 to the plank arranged for the game. Also compare teeter-totter under teeter (v.). 

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