Etymology
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pooh-pooh (v.)

"to dismiss lightly and contemptuously," literally "to turn aside with an exclamation of 'pooh,'" 1827, a slang reduplication of dismissive expression pooh. Among the many 19th century theories of the origin of language was the Pooh-pooh theory (1860), which held that language grew from natural expressions of surprise, joy, pain, or grief.

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pooh 

1590s, "a 'vocal gesture' expressing the action of puffing anything away" [OED], used as an exclamation of dislike, scorn, or contempt, first attested in Hamlet Act I, Scene III, where Polonius addresses Ophelia with, "Affection! pooh! you speak like a green girl, / Unsifted in such perilous circumstance. / Do you believe his tenders, as you call them?" But the "vocal gesture" is perhaps ancient.

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pooh-bah (n.)

"leader who maintains excessive bureaucratic control," 1888, from Pooh Bah, the name of the "Lord High Everything Else" character in Gilbert and Sullivan's "The Mikado" (1885).

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poo (n.)
also pooh, baby-talk for "excrement," 1950s (see poop (n.2)).
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