Archaic second-person singular past tense of 'can'; what you are able to do in old English.
From Old English 'cuðe,' the past tense of 'can,' with the archaic verb ending '-est' for second-person singular (thou couldest = you could).
English used to have different verb endings based on who you were talking to—'thou couldest' was intimate, while 'ye could' was formal, and we lost this entirely!
Complete word intelligence in one call. Free tier — 50 lookups/day.