This is as obscure as any word that has featured here. If you think it looks Latin, you’re right. It’s the the ablative plural of the present participle of the verb recumbere, to recline or rest (it’s also the source of English recumbent).
In Roman times, recumbentibus meant more than just reclining. It appears twice in St Mark’s gospel in the Vulgate, the Latin bible that was compiled in the fourth century. This is from verse 16:14: “novissime recumbentibus illis undecim apparuit”. This is translated in the King James Bible as “Afterward he appeared unto the eleven as they sat at meat”. More recent translations render recumbentibus in the words “as they sat at table” or “as they were eating”. Its sense is explained by Romans liking to recline on couches to eat their meals.
Educated Englishmen of late medieval times, who all knew Latin, were well acquainted with recumbentibus and generated from it a witty meaning of forcing somebody to adopt a reclining position, in particular by a powerful blow. In his Dialogue of Proverbs of 1549, John Haywood wrote this, “Had you some husband, and snapped at him thus, I wise he would give you a recumbentibus.” Thomas Middleton has a character in his play of 1608, The Family of Love, exclaim, “A plague upon him for a Glister! He has given our loves a suppositor with a recumbentibus.” He is complaining about an unnecessarily violent and embarrassing medical procedure, since we would now call a suppositor a suppository and glister is another way to write clyster, a medicine injected into the rectum to empty or cleanse the bowels.
Recumbentibus has never become common in English and has in any case been obsolete since about 1670, though it has on very rare occasions been resurrected:
Thor went among them with incalescent eagerness, smashing their guidance systems with his bare fingers, delivering one massive recumbentibus after another, making shards of the casings.
And Another Thing ..., by Eoin Colfer; part six of Douglas Adams’s trilogy The Hitchhiker’s Guide to the Galaxy, 2009. Incalescent means with increasing heat.