could: [16] Could began life as cūthe, the past tense of Old English cunnan ‘can’. By Middle English times this had become couthe, and in the 14th century it developed to coude under the influence of the standard past tense ending (e)de. The l was introduced in the 16th century, to bring coude into line with would and should. => kith, uncouth
Old English cuðe, past tense of cunnan "to be able" (see can (v.1)); ending changed 14c. to standard English -d(e). The excrescent -l- was added 15c.-16c. on model of would, should, where it is historical.
