Like some Polish guys ran into some Germans, and the Germans just started speaking some gibberish non-slavic language. Polish guys were like "I really doubt this is a language. They must not be able to talk at all."
Simply because Slavians share common roots of the language and can more or less communicate with each other while Germanic tribes were 'non-speakers'.
Comanche (from a Ute word meaning "they fight with us.")
Apache (from a Zuni word for "enemy.")
"The term Maliseet is the exonym by which the Mi'kmaq people referred to this group when speaking to early Europeans. Maliseet was a Mi'kmaq word meaning "broken talkers", "lazy speakers" or "he speaks badly," or differently by which the Mi'kmaq contrasted the other tribe's language to their own.[3] The Wolastoqiyik and Mi'kmaq languages are closely related but distinctly different. The Europeans met the Mi'kmaq before the Wolastoqiyik, and adopted their term of Malesse'jik (transliterated as Malécite in French) for the people, not understanding that it was not their true name. The later English colonists anglicized this term as Maliseet, in another transliteration of sound."