Potassium

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early 19th century: from potash (earlier potass, from French potasse ) + -ium.

wiktionary

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Coined by British chemist Humphry Davy in 1807, from  potassa +‎  -ium. 

etymonline

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potassium (n.)

metallic element, 1807, coined by English chemist Sir Humphry Davy from Modern Latin potassa, Latinized form of potash (q.v.). Davy first isolated it from potash. The chemical symbol K is from Latin kalium "potash," from Arabic al-qaliy "the ashes, burnt ashes" (see alkali). Related: Potassic.