Eighty

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Old English hunde(a)htatig, from hund (of uncertain origin) + e(a)hta ‘eight’ + -tig (see -ty2); the first element was lost early in the Middle English period.

wiktionary

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From Middle English eiȝti, eyȝty, eiȝtety, aghty, from Old English hundeahtatiġ, from Proto-Germanic *ahtōutēhundą(“eighty”), equivalent to eight +‎ -ty. Cognate with Scots hechty, auchty(“eighty”), Saterland Frisian tachentich(“eighty”), West Frisian tachtich(“eighty”), Dutch tachtig(“eighty”), German Low German tachentig(“eighty”), German achtzig(“eighty”), Swedish åttio(“eighty”), Norwegian åtti(“eighty”), Icelandic áttatíu(“eighty”).

etymonline

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eighty (adj., n.)

"1 more than seventy-nine, twice forty; the number which is one more than seventy-nine; a symbol representing this number;" late 13c., eigteti, from eight + -ty (1). Replacing Old English hundeahtatig, with hund- "ten." Related: Eightieth.