Way back in medieval times there was the English cokewold, which eventually became our modern cuckold. The first bit is also the source of our cuckoo, with –old attached as a suffix giving a pejorative sense.
Some clever wordsmith of the fifteenth century took the old word and changed the first part into wete, the quality of awareness or knowledge, so making wetewold. Wete is our modern wit in slight disguise. So the idea behind wittol — as it was later written and said — was of somebody who knew he was a cuckold, but especially somebody who was untroubled by the situation.
This was not a state of affairs that most spectators thought reasonable or likely, so the word later took on a sense of a half-witted person, a fool. It was not accidental that William Congreve entitled a character in his play The Old Bachelor Sir Joseph Wittol. Shakespeare borrowed the adjective in The Merry Wives of Windsor: “I know him not; yet I wrong him to call him poor; they say the jealous wittolly knave hath masses of money; for the which his wife seems to me well-favoured”.
The word has pretty much gone out of use, and most old books in which you will find the word use it of a fool, but a few writers retained the cuckold sense as late as the nineteenth century. Here’s an example from an American literary magazine of 1840: “I knew full well that the wittol husband is a subject of ridicule rather than sympathy, and therefore, carefully concealed my suspicions”.