1712, from addle (n.) "urine, liquid filth," from Old English adela "mud, mire, liquid manure" (cognate with Old Swedish adel "urine," Middle Low German adel, Dutch aal "puddle").
Used in noun phrase addle egg (mid-13c.) "egg that does not hatch, rotten egg," literally "urine egg," a loan-translation of Latin ovum urinum, which is itself an erroneous loan-translation of Greek ourion oon "putrid egg," literally "wind egg," from ourios "of the wind" (confused by Roman writers with ourios "of urine," from ouron "urine"). Because of this usage, from c.1600 the noun in English was taken as an adjective meaning "putrid," and thence given a figurative extension to "empty, vain, idle," also "confused, muddled, unsound" (1706). The verb followed a like course. Related: Addled; addling.