Douglas Harper's Etymology Dictionary
French, à deux, literally "for two" (see deuce).
Usage examples of "a deux".
They exist in a folie a deux in which they share the same delusions.
You-didn't-ask-me-but-I'm-telling-you-for-your-own-good department: Medical science chronicles cases of cancer a deux, which is the contraction of cancer simultaneously or consequently by two persons who live together.
My dear Kitty told me today that she would dance in a deluge before ever she would starve in such an ark of salvation for, as she reminded me (blushing piquantly and whispering in my ear though there was none to snap her words but giddy butterflies), dame Nature, by the divine blessing, has implanted it in our hearts and it has become a household word that IL Y A DEUX CHOSES for which the innocence of our original garb, in other circumstances a breach of the proprieties, is the fittest, nay, the only garment.
My dear Kitty told me today that she would dance in a deluge before ever she would starve in such an ark of salvation for, as she reminded me (blushing piquantly and whispering in my ear though there was none to snap her words but giddy butterflies), dame Nature, by the divine blessing, has implanted it in our hearts and it has become a household word that il y a deux choses for which the innocence of our original garb, in other circumstances a breach of the proprieties, is the fittest, nay, the only garment.