mephitic [muh-fit-ik]

adjective:
1 offensive to the smell
2 noxious; pestilential; poisonous

Examples:

Like a mephitic vapor from a sword-and-sandals epic, it slips under the door frame and into your head. (Guy Trebay, We’re Holding Tight to Our Good Luck Talismans, The New York Times, April 2020)

These moments of reckoning - in which something that once felt exciting begins to seem noxious, mephitic, dangerous - are important to heed. (Amanda Petrusich, A Quest to Rename the Williamsburg Bridge for Sonny Rollins, The New Yorker, April 2017)

The A66 motorway takes you along the bank of a river that eventually opens into the Cantabrian Sea, but there's no water to be seen through a mephitic landscape of factories and warehouses. (Paul Richardson, A great white hope in Avilés, Asturias, The Guardian, July 2011)

Mephitic vapors - spontaneous combustion - pressure of gases born of long decay - any one of numberless phenomena might be responsible. (H P Lovecraft, 'The Haunter of the Dark')

I even made them remove from the opening, as I smelled the mephitic air that issued abundantly from it, and began myself to feel giddiness in consequence of having gone too near; so that I was compelled to withdraw quickly, and inhale a purer air. (Johann David Wyss, The Swiss Family Robinson)

Origin:
1620s, 'of poisonous smell, foul, noxious,' from Late Latin mephiticus, from Latin mephitis, mefitis 'noxious vapor, a pestilential exhalation, especially from the earth' (also personified as a goddess believed to have the power to avert it), an Italic word of uncertain origin. English use of mephitis is attested from 1706. (Online Etymology Dictionary)

sempiternal [sem-pi-tur-nl]

adjective:
(literary) everlasting; of never-ending durationeternal

Examples:

Must we imagine Sisyphus to be happy, as Albert Camus proposed? Or would a sempiternal - an eternal, unchanging - life ultimately lack any purpose? (Johanna Thomas-Corr, Help! I’m trapped in Groundhog Day, the novel, The Times, April 2025)

Fires raged and floods drove through streets and houses as the planet became more and more inimical to human life. The sempiternal nurdles, indestructible, swayed on and under the surface of the sea. (A S Byatt, Sea Story, The Guardian, March 2013)

I certainly didn't suspect a number of things: that I'd be soundly beaten by my teenage son; that shortly thereafter I'd become obsessed with table tennis; that my obsession would fuel a grueling initiation that, in a sense, is still going on today; that the sport itself would reacquaint me with some eternal principles of the Perennial Philosophy and afford me new glimpses of sempiternal wisdom; that it would teach me so much about myself, our human condition, and life; and that, finally, in 'humble' table tennis I'd be looking for the living presence that informs the phenomenal world. (Guido Mina Di Sospiro, The Metaphysics of Ping Pong)

A living shell in which its tenant lay dormant, her subjective will to live alone kept this woman going her sempiternal rounds of monotony. (Louis Joseph Vance, Joan Thursday)

He wrote: "Isn't that lovely and tear-drawing? true and tender and sempiternal?" And then he copied out the whole song, in case I should chance not to have the text at hand. (Baron Hallam Tennyson Tennyson, Tennyson and his friends)

Origin:
'eternal and unchanging, perpetual, everlasting, having no end,' early 15c, from Old French sempiternel 'eternal, everlasting' (13c) or directly from Medieval Latin sempiternalis, from Latin sempiternus 'everlasting, perpetual, continual,' from semper 'always, ever'. The earlier Middle English adjective was sempitern (late 14c) from Old French sempiterne and Latin sempiternus. (Online Etymology Dictionary)

Despite their similarities, sempiternal and eternal come from different roots. Sempiternal is derived from the Late Latin sempiternalis and ultimately from semper, Latin for 'always.' Eternal, on the other hand, is derived, by way of Middle French and Middle English, from the Late Latin aeternalis and ultimately from aevum, Latin for 'age' or 'eternity.' Sempiternal is much less common than eternal, but some writers have found it useful. 19th-century American writer Ralph Waldo Emerson, for example, wrote, 'The one thing which we seek with insatiable desire is to forget ourselves, … to lose our sempiternal memory, and to do something without knowing how or why….' (Merriam-Webster)