The plural form of the expression “at sixes and sevens” is comparatively modern, dating back only a hundred and fifty years or so.
The older form, “on six and seven,” however, was so old and well known in Chaucer’s day that, worse luck, he didn’t bother to explain what it meant when, about 1375, he used it in Troylus and Cryseyde.
As we use it today and as it has been used for centuries, the phrase means “in a state of disorder or confusion; topsy-turvy.” Explanations of its origin have been sought, but nothing certain is known.
One writer tries to connect it with a Hebrew phrase that we find in Job v, 19, “He shall deliver thee in six troubles: yea, in seven there shall no evil touch thee.” Another seeks an explanation in the Arabic numerals 6 and 7, which, he points out, extend higher and lower respectively in a line of figures than do the others; hence, that these two are irregular.
But it is more probable that Chaucer’s use had reference to an old dicing game. From Chaucer and other old sources we know of one game in which to try a throw of a five and a six (cinque and sice were the old names) was regarded as the most risky gamble to be made.
One who staked his chance on such a throw was reckless in the extreme, utterly careless of consequences. To hazard such a throw was “to set on cinque and sice,” in the old wording. It is presumed that Chaucer’s use, “to set on six and seven,” had reference to a similar game.
From heedlessness and carelessness in taking such a risk, the expression “on six and seven,” later changed to “at sixes and sevens,” may have come to denote general carelessness; hence, disorder and confusion.