acid-fast

adjective

ac·​id-fast ˈa-səd-ˌfast How to pronounce acid-fast (audio)
: not easily decolorized by acids

Word History

First Known Use

1903, in the meaning defined above

Time Traveler
The first known use of acid-fast was in 1903

Dictionary Entries Near acid-fast

acid egg

acid-fast

acid fog

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Cite this Entry

“Acid-fast.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/acid-fast. Accessed 30 Mar. 2023.

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Kids Definition

acid-fast

adjective
ac·​id-fast
ˈas-əd-ˌfast
: not easily made to lose color by acids

Medical Definition

acid-fast

adjective
ˈas-əd-ˌfast How to pronounce acid-fast (audio)
: not easily decolorized by acids (as when stained)
—used especially of bacteria and tissues
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