infiltrate

Etymology

It is formed from English in- + filtrate,^{[2][3][4][5][6][7]} perhaps after French infiltrer, used by Paré in the 16th century.^[2][3] The second part is from Medieval Latin filtrātus, the past participle of filtrō (“to filter”), from filtrum (“felt”), the source of filter.^[3][2] The Oxford Dictionary of English notes that the noun infiltration came first, in Middle English;^[8] Etymonline records it in a medical sense, a knitting together, in the early 15th century, from infiltrātiō.^[3]

Earliest recorded meaning

To introduce a fluid by filtration, or to pass into a substance through its pores: 1739, in J. Sparrow's translation of Le Dran's Observations in Surgery;^[2] Etymonline and Merriam-Webster date the verb to 1758.^[3][6]

Semantic evolution

For nearly two centuries the verb was used of fluids passing into or through the pores of a solid, in physics, geology and medicine; the intransitive use, to percolate, is recorded from 1828, in Webster's dictionary.^[2] The military sense, to move troops a few at a time through gaps in the enemy's line, is first recorded in 1934, in Webster's New International Dictionary,^[2][3] and the noun infiltration had been used in the same sense since 1930.^[3] From there the verb was extended in the mid 20th century to entering an organization or group secretly and by degrees.^[7][6] The noun infiltrate, chiefly medical, for a substance that has passed into the tissues, dates from the late 19th century.^[7][6]

Timeline

• 1739 – to introduce a fluid by filtration; to permeate a substance through its pores.^[2] Etymonline dates the word to 1758.^[3]
• 1828 – intransitive, to percolate through pores.^[2]
• late 19th century – noun, chiefly in medicine, an infiltrated substance.^[7]
• 1934 – military, to penetrate enemy lines by moving small groups of troops through them.^[2][3]
• mid 20th century – to enter an organization or territory secretly and gradually.^[7]

Meanings

Verb

1. To enter or join (a group, organization or place) secretly and gradually, especially in order to spy or to influence it: Undercover officers infiltrated the smuggling ring.^[5][6][7]
2. In military use, to pass troops a few at a time, undetected, through (an enemy line or position).^[5][6][2]
3. To pass into or through (a substance) by filtering through its pores; to permeate.^[5][6][2]
4. To introduce (a fluid) into something by such means, as an anesthetic into tissue.^[6][2][7]

Noun

1. Something that infiltrates; in pathology, a substance that passes into and accumulates in cells or tissues.^[5][6][7]
2. A local anaesthetic solution injected into the tissues.^[5]

Word family

• infiltrator
• infiltration
• infiltrative
• uninfiltrated
• reinfiltrate

Related words

• filter
• filtrate
• felt

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f g h i Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
4. ^ Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
5. ^ a b c d e f Collins English Dictionary (2023), Glasgow: HarperCollins
6. ^ a b c d e f g h i Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
7. ^ a b c d e f g h Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
8. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press