Dolt

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mid 16th century: perhaps a variant of dulled, past participle of dull.

wiktionary

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First used as a noun in Early Modern English, from dialectal English dold(“stupid, confused”), from Middle English dold, a variant of dulled, dult(“dulled”), past participle of dullen, dollen(“to make dull, make stupid”), from dull, dul, dwal(“stupid”). More at dull.

etymonline

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dolt (n.)

"dull, stupid fellow," 1540s, perhaps a variant of dold "dull, foolish" (mid-15c.), influenced by dulte, dolte, past-participle forms of Middle English dullen "to dull; make or become dazed or stupid" (see dull (v.)). Related: Doltish "foolish, stupid" (1540s); doltishly; doltishness.