Lying on the ground, especially in penitence or humiliation.
Once again we are in the realm of inconsequential words whose tenuous hold on existence is maintained by people who create lists of obscure words for our enjoyment and edification. Many writers and online dictionaries define it simply as “lying on the ground”, but on the few occasions on which it has been used in real life it has always had associations with religion.
That’s the result of its first, and perhaps its most significant, appearance. That was in one of a series of tracts written in the 1650s by John Bramhall, then Bishop of Derry in Ireland, opposing the views of the English materialist philosopher Thomas Hobbes. Bramhall wrote: “He is afraid, that ‘this doctrine’ of fasting, and mourning, and tears, and humicubation, and sackcloth, and ashes, ‘pertaineth to the establishment of Romish penance.’”
But in its etymology humicubation has no reference to penitence. It comes from the Latin words humi, on the ground, plus cubare, to lie down. The first bit is closely related to Latin humus, which we have taken over as the name for the organic component of soil. The second element is the source of cubicle (originally a bedchamber, a place in which one lies down), and concubine (a person with whom one euphemistically lies down).