acid-fast

adjective

ac·id-fast ˈa-səd-ˌfast

: 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

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“Acid-fast.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/acid-fast. Accessed 22 May. 2025.

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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

: not easily decolorized by acids (as when stained)

—used especially of bacteria and tissues

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acid-fast

adjective

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