unmitigable

the unmitigable environmental impact of the proposed project

[T]hou [Ariel] vvaſt a Spirit too delicate / To act her [the witch Sycorax's] earthy, and abhord commands, / Refuſing her grand heſts, ſhe did confine thee / By helpe of her more potent Miniſters, / And in her moſt vnmittigable rage, / Into a cloven Pyne, […]

1610–1611 (date written), William Shakespeare, “The Tempest”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act I, scene ii], page 4, column 1

Oh for some few months of firmer health! This unmitigable disorder, which chains me to the chamber and the chair!

1780, Samuel Jackson Pratt, Emma Corbett: or, The Miseries of Civil War, Bath: Pratt and Clinch, Volume 3, Letter 111, pp. 40-41

[…] Sleep, impiteous sleep, / Unmitigable, uncorruptible gaoler, / Come, cloak my senses with thy leaden robe,

1911, Maurice Baring, “The Death of Alexander”, in Diminutive Dramas, London: Constable, page 36