Meaning

noun a person who is not very bright

originFirst 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.

Etymology from Wiktionary, CC BY-SA 3.0