Hydrant

google

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early 19th century (originally US): formed irregularly from hydro- ‘relating to water’ + -ant.

wiktionary

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An irregular formation: hydr- +‎ -ant.

etymonline

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hydrant (n.)

"apparatus for drawing water from a street main," 1806, from Greek hydr-, stem of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet") + -ant. OED double-damns it as "Irregularly formed" and "of U.S. origin."