The term field bishop denotes a person who is hanged and imagined as grotesquely giving a benediction with his jerking legs.

It is first recorded in A mysterye of inyquyte contayned within the heretycall genealogye of Ponce Pantolabus (1545), by John Bale (1495-1563), Bishop of Ossory, evangelical polemicist and historian:

What your ende shall be / the Lorde of heauen knoweth. ye haue yet layser [= leisure] ynough to playe the felde bysshop / and to blesse with youre heles in an hempen corde / as a great sort of youre fellawes haue done which were as true menne as you are here.

It corresponds to French évêque des champs, literally bishop of the fields, which was used for example by the French satirist François Rabelais (circa 1494-1553) in Pantagrueline prognostication (Pantagruelian Prognostication), which parodies and subverts works of astrological divination:

(1535 edition)
A Mars comme bourreaux, meurtriers, adventuriers, brigans, sergeans, records de tesmoings, gens de guet, mortepayes, arracheurs de dens, coupeurs de couilles, barberotz, bouchiers, faulx monnoieurs, medicins de trinquenique, tacnins & marranes, renieurs de dieu, allumetiers, boutefeux, ramonneurs de cheminée, franctaupins, charbonniers, alchimistes, coquassiers, grillotiers, chercuitiers, bimbelotiers : manilliers, lanterniers, maignins feront ceste année de beaux coups, mais aulcuns d’iceulx seront fort subiectz à recepvoir quelque coup de baston à l’emblée. Un des sudictz sera ceste année faict evesque des champs donnant la benediction avec les pieds aux passans.

translation by Pierre Antoine Motteux (1663-1718) – 1694 edition:
Those who are under Mars, as Hang-men, Cut-throats, Dead-going Fellows, Free-booters, Hedge-birds, Foot-pads and Highway-men, Catchpoles, Bum-bailiffs, Beadles and Watch-men, Reformado’s, Tooth-drawers and Corn-cutters, Pintle-smiths, Shavers and Frig-beards, Butchers, Coiners, Paultry-Quacks and Mountebanks, Renegado’s, Apostates, and Marraniz’d Miscreants, Incendiaries or Boutefeu’s, Chimney-sweepers, Boorish Clusters-fists, Charcoal-men, Alchymists, Merchants of Eel-skins and Egg-shells, Grid-iron and Rattle-makers, Cooks, Paultry-Pedlers, Trash-mongers and Spangle-makers, Bracelet-makers, Lantern-makers and Tinkers, this Year will do fine things; but some of them will be somewhat subject to be Rib-roasted, and have a St. Andrew Cross scor’d over their Jobbernols at unawares. This year one of those Worthy Persons will go nigh to be made a Field-Bishop, and, mounted on a Horse that was foal’d of an Acorn, give the Passengers a Blessing with his Legs.

In the same book, Rabelais wrote this well-known passage:

This Year the Stone-blind shall see but very little; the Deaf shall hear but scurvily; the Dumb shan’t speak very plain; the Rich shall be somewhat in a better case than the Poor, and the Healthy than the Sick.
original text:
Ceste année les aveugles ne verront que bien peu, les sourdz oyront assez mal : les muetz ne parleront guières : les riches se porteront un peu mieulx que les pauvres, & les sains mieulx que les malades.