on pain of

From on + pain (“suffering inflicted as punishment or penalty”) + of;[1] compare French sous peine de.

• (Received Pronunciation) IPA^(key): /ɒn ˈpeɪn əv/
• (General American) IPA^(key): /ɑn ˈpeɪn əv/

on pain of

1. Used to warn of consequences if a specified act is done or not done: subject to the punishment of (some penalty); or (generally) subject to the circumstance of (some thing).

   Synonyms: under pain of, upon pain of

   • 1595 December 9 (first known performance), [William Shakespeare], The Tragedie of King Richard the Second. […] (First Quarto), London: […] Valentine Simmes for Androw Wise, […], published 1597, →OCLC, [Act I, scene iii]:

     On paine of death, no perſon be ſo bold, / Or daring, hardy, as to touch the liſtes, / Except the Martiall and ſuch officers / Appoynted to direct theſe faire deſignes.

   • [1629 (first performance), B[en] Jonson, The New Inne. Or, The Light Heart. […], London: […] Thomas Harper, for Thomas Alchorne, […], published 1631, →OCLC, Act II, scene vi:

     [N]o flattery for't: / No lick-foote, paine of looſing your proboſcis: […]]

   • 1690, [John] Dryden, Don Sebastian, King of Portugal: […], London: […] Jo. Hindmarsh, […], →OCLC, Act IV, page 105:

     Seb[astian]. […] [M]y duty, then, / To interpoſe; on pain of my diſpleasure, / Betwixt your Swords[.] / Dor[ax]. On pain of Infamy / He ſhould have diſobey'd.

   • 1912 October, Edgar Rice Burroughs, “Tarzan of the Apes”, in The All-Story, New York, N.Y.: Frank A. Munsey Co., →OCLC; republished as “Out to Sea”, in Tarzan of the Apes, New York, N.Y.: A. L. Burt Company, 1914 June, →OCLC, page 16:

     Translated, it was a warning to the Claytons to refrain from reporting the loss of the revolvers, or from repeating what the old sailor had told them—to refrain on pain of death.

   • 1943 January, Irving Chernev, “Chess Thrillers”, in Chess Review:

     The White Queen is offered for sacrifice six times in succession, and must be refused each time on pain of checkmate!

• at the risk of

• “on/under pain of death”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “on/under pain of death” in Longman Dictionary of Contemporary English, Longman.
• “on/under pain of something” (US) / “on/under pain of something” (UK) in Macmillan English Dictionary.