Kleenex

wiktionary

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From the name of brand of tissues trademarked by Cellucotton Products Company (now Kimberly-Clark) in 1924–25. Alteration of clean + -ex.

etymonline

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Kleenex (n.)

1924, proprietary name, registered by Cellucotton Products Company, Neenah, Wisconsin, U.S.; later Kimberly-Clark Corp. An arbitrary alteration of clean + brand-name suffix -ex.