Gibberish

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early 16th century: perhaps from gibber1 (but recorded earlier) + the suffix -ish1 (denoting a language as in Spanish, Swedish, etc.).

wiktionary

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ca. 16th century. Either an onomatopoeia, imitating to the sound of chatter, probably influenced by jabber, or derived from the root of the Irish gob(“the mouth”). [1]

etymonline

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gibberish (n.)

"rapid and inarticulate speech; talk in no known language," 1550s, imitative of the sound of chatter, probably influenced by jabber. Used early 17c. of the language of rogues and gypsies.