rarefy

Etymology

It is inherited from Middle English rarefien,^[2][3][4][5] partly borrowed from Middle French rarefier (“to make less dense”) and partly borrowed from Latin rārēfaciō (“to make less solid”),^[6] with rārificō as a further source according to Barnhart and Etymonline.^[3][4] The Latin verb is from rārus (“rare, thin”) and faciō (“to make”),^[6][7][4] the second part from Proto-Indo-European *dhe- (“to set, put”).^[4] The OED suggests that the form, with -ē- rather than the expected -i-, follows ārefaciō,^[6] while Dictionary.com calls the origin of its long -ē- unclear.^[8] Merriam-Webster Unabridged takes the French ending as influenced by the suffix -fier, and derives the less common spelling rarify from the Medieval Latin verb.^[5] The first vowel was originally short in Middle English and changed through association with rare.^[6]

Earliest recorded meaning

To make thin or less dense: before 1398, in Trevisa's translation of Bartholomaeus Anglicus, where heat "rarefieþ and openeþ þe poores".^[2][6][3] In this first use it applied to the flesh and its pores as well as to matter generally.^[2]

Semantic evolution

Its first uses concern the pores of the flesh and matter in general, and it has been used especially of air.^[2][6] Figurative use followed around 1600 with Ben Jonson's Every Man out of his Humour, for making a thing finer or purer;^[6][3] Barnhart dates the play to 1599 and the OED its first quotation to 1600.^[3][6] From 1667 to 1875 the word also meant making an idea or argument more subtle, usually to its harm.^[6] The intransitive sense "to become less dense" came late, in the 1650s.^[6][4] Johnson (1755) gave both the transitive "to make thin: contrary to condense" and the intransitive "to become thin", and thought rarify the more proper spelling.^[9] A 20th-century use, to talk in a lofty manner, is first recorded in E. Blunden's Undertones of War (1928).^[6]

Timeline

• before 1398 – to make thin or less dense.^[2][6][3]
• 1600 – to make less material; to refine or purify.^[6] Barnhart has 1599 for the same use in Jonson.^[3]
• 1637 – to extenuate a fault (obsolete, rare).^[6]
• 1650 – to thin the trees of a wood (obsolete by 1661).^[6]
• before 1658 – intransitive, to become less dense.^[6]
• 1667 – to make an idea or argument more subtle (obsolete by 1875).^[6]
• 1928 – intransitive, to talk in a lofty manner.^[6]

Meanings

Verb

1. To make thin or less dense, especially by expansion; used especially of air: Altitude rarefies the atmosphere.^[10][5][6]
2. To become thin or less dense.^[10][5][6]
3. To make less material or more subtle; to refine or purify.^[5][6][11]
4. To talk in a lofty or exalted manner.^[6]

Word family

• rarefiable
• rarefier

Related words

• rarefaction
• rare

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e Kurath, Hans; Kuhn, Sherman M.; Lewis, Robert E., eds. (1952–2001), Middle English Dictionary, Ann Arbor: University of Michigan Press; online edition ed. Frances McSparran et al. (2000–2018)
3. ^ a b c d e f g Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
4. ^ a b c d e Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
5. ^ a b c d e Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ a b c d e f g h i j k l m n o p q r s t u v Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
7. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
8. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
9. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
10. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins
11. ^ Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press