Sixty

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Old English siextig (see six, -ty2).

wiktionary

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From Middle English sixty, sexty, Old English siextiġ, from Proto-Germanic *sehs tigiwiz(“sixty”), equivalent to six +‎ -ty. Cognate with Scots sexty, saxty(“sixty”), Saterland Frisian säkstich(“sixty”), West Frisian sechstich(“sixty”), Dutch zestig(“sixty”), German Low German sesstig(“sixty”), German sechzig(“sixty”), Swedish sextio(“sixty”), Norwegian seksti(“sixty”), Icelandic sextíu(“sixty”). Compare also Sanskrit षष्टि(ṣaṣṭi).

etymonline

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sixty (adj., n.)

"1 more than fifty-nine, twice thirty; the number which is one more than fifty-nine; a symbol representing this number;" Old English sixtig, from siex (see six) + -tig (see -ty (1)). Similar formation in Old Norse sextugr, sextögr, sextigir, Old Frisian sextich, Middle Dutch sestig, Dutch zestig, Old High German sehszug, German sechzig. Phrase sixty-four dollar question is attested from 1942, from a radio quiz show where that was the top prize.