The phrase at sixes and sevens means in a state of total confusion or disarray.

Based on the language of dicing, the phrase was originally to set (all) on six and seven. It denoted the hazard of one’s whole fortune, or carelessness as to the consequences of one’s actions. From this earlier association with reckless behaviour came the idea that things in disorder were at sixes and sevens.

The pips on a die, and later on playing cards, used to be numbered in an approximation to French: ace (which is still used in card-playing), deuce, trey (both of which have persisted), quatre, cinq(ue) and sice. According to a popular explanation, to set (all) on six and seven is an alteration of another phrase, to set (all) on cinque and sice, which meant to gamble on the highest numbers, and figuratively to behave recklessly. For example, the Scottish chronicler and poet William Stewart (floruit 1499-1541) wrote, in The Buik of the Croniclis of Scotland (1535):

And greit folie to set on synk and syss.

And the English playwright and satirist Ulpian Fulwell (1546?-1586?) wrote, in the play titled Like Will to Like (1568):

All that I had is now lost at the dice,
My sword, my buckler, and all at sink and cise.

The popular explanation is that, in the course of time, the numbers six and seven were substituted for cinque and sice, perhaps because the literal meanings of the original words were forgotten: cinque, pronounced sink, would have been incorrectly anglicised as six, so sice became seven, and the whole phrase gradually assumed its familiar form.

Those who spread this rather far-fetched theory content themselves with merely reproducing, and presenting as a fact, what is only a supposition in the Oxford English Dictionary (1^st edition, 1911). But the phrase to set (all) on six and seven seems to have been completely independent from to set (all) on cinque and sice. The latter is attested in the early 16^th century, whereas the former is first recorded in the late 14^th century, when the English poet Geoffrey Chaucer (circa 1342-1400) wrote, in Troilus and Criseyde:

Kith now somwhat thi corage and thi myght;
Have mercy on thiself for any awe.
Lat nat this wrecched wo thyn herte gnawe,
But manly sette the world on six and sevene;
And if thow deye a martyr, go to hevene!
     translation:
Show now somewhat your courage and strength. Fear not, but have mercy on yourself. Let not this wretched woe gnaw upon your heart, but stake the world like a man on the cast of the dice, and if you die as a martyr, go to heaven!

It is impossible that the more recent phrase, to set (all) on cinque and sice, is the origin of the earlier to set (all) on six and seven.

Interestingly, another obsolete phrase, to set (all) on seven, meant to make a desperate venture, to rush to battle. For example, in the Alliterative Morte Arthure (King Arthur’s Death), written about 1400, there is:

With that steelen brand   he stroke off his hed.
Sterenly in that stour   he strikes another.
Thus he settes on seven   with his seker knightes;
Whiles sixty were served so   ne sesed they never;
And thus at this joining   the giauntes are destroyed,
And at that journee for-jousted   with gentle knightes.
     translation:
And with his steel sword he struck off his head.
Stoutly into that struggle he strikes at another,
And sets on seven with his stalwart knights –
Till sixty were so served, ceased they never.
And thus in that skirmish the giants are slain,
Laid low in that battle by lordly knights.

The phrase to set (all) on seven was originally used with reference to the throwing of a main in the game of hazard, a game at dice in which the chances were complicated by a number of arbitrary rules. The following description of this game is from The Encyclopædia Britannica (1892):

The player or “caster” calls a “main” (that is, any number from five to nine inclusive). He then throws with two dice. If he “throws in,” or “nicks,” he wins the sum played for from the banker or “setter.” Five is a nick to five, six and twelve are nicks to six, seven and eleven to seven, eight and twelve to eight, and nine to nine. If the caster “throws out” by throwing aces, or deuce, ace (called crabs), he loses. When the main is five or nine the caster throws out with eleven or twelve; when the main is six or eight he throws out with eleven; when the main is seven he throws out with twelve. If the caster neither nicks nor throws out, the number thrown is his “chance,” and he keeps on throwing till either the chance comes up, when he wins, or till the main comes up, when he loses. When a chance is thrown, the “odds” for or against the chance are laid by the setter to the amount of the original stake.

To set (all) on six and seven, to set (all) on cinque and sice and to set (all) on seven were probably independent phrases. They all implied abandon to chance, as in the following note from the 1542 translation of Erasmus’s apophthegms by the English schoolmaster and playwright Nicolas Udall (1504-56):

There is a prouerbe, omnem iacere aleam, to cast all dyce by whiche is signified, to sette all on sixe & seuen, & at all auentures to ieoperd assaiyng [= to risk attempting] the wilde chaunce of fortune, bee it good bee it badde.

Similarly, in his Latin-English Thesaurus Linguae Romanae & Britannicae (1573), Thomas Cooper wrote:

Aleam omnem iácere. Sueton. To put in aduenture: to set at sixe and seuen.

And Randle Cotgrave wrote, in A Dictionarie of the French and English Tongues (1611):

Desesperade: feminine. A kind of mournefull song.
Iouër à la desesperade. To set his whole rest, or set all on sixes, and sevens; to throw at all.