Etymology The noun is derived from Latin nōn plūs (“no further, no more”), from nōn (“not”) + plūs (“additionally, more; further”) (ultimately from Proto-Indo-European *pleh₁- (“to fill”)). The verb is derived from the noun.
[A]ltering Vaudemont, to Vallemontanus, and metamorphoſing them, by ſuting them to the Græcian or Latin tongue, we know not what to make of them, and are often at a non-plus.1603, Michel de Montaigne, “Of Names”, in John Florio, transl., The Essayes […], book I, London: […] Val[entine] Simmes for Edward Blount […], →OCLC, pages 149–150:
Being now made much abler to make their queries, of the ſecrets of that myſtery, by how much their often failings, had put them to often ſtops and nonpluſſes in the work.1657, Richard Ligon, A True & Exact History of the Island of Barbados. […], London: […] Humphrey Moseley, […], page 85:
2 more examples
And if both of them are at a perfect Non-plus, and Baffle to all Humane Underſtanding; is it poſſible for Natural Reaſon to comprehend what the Heart of Man cannot conceive?1715, Robert South, “A Discourse Concerning the General Resurrection. On Acts xxiv. 15. […]”, in Twelve Sermons Preached at Several Times, and upon Several Occasions, volume IV, London: […] G. James, for Jonah Bowyer […], →OCLC, page 269:
I believe they'd ſoon be put to a non-plus—You'd be quite too much for them, I'm ſure.1792, Charlotte Smith, “Letter VI. To Mr. Bethel.”, in Desmond. […], volume I, London: […] G[eorge,] G[eorge,] J[ohn] and J[ames] Robinson, […], →OCLC, page 89:
Etymology The noun is derived from Latin nōn plūs (“no further, no more”), from nōn (“not”) + plūs (“additionally, more; further”) (ultimately from Proto-Indo-European *pleh₁- (“to fill”)). The verb is derived from the noun.
1
To bewilder or perplex (someone); to confound, to flummox.
You couldn’t have told it from my manner, but I was feeling more than a bit nonplussed. The spectacle before me was enough to nonplus anyone.1934 October 5, P[elham] G[renville] Wodehouse, chapter II, in Right Ho, Jeeves, London: Herbert Jenkins […], →OCLC, page 24: