adjective
an adjective describes or modifies a noun or pronoun, in this case 'overfamiliar' describes someone who is excessively familiar or informal
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The employee's overfamiliar attitude towards their supervisor was deemed unprofessional.
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Don't get too overfamiliar with your new classmates right away; it's better to take things slow.
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She's always been a bit too overfamiliar with people she just met.
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The overfamiliar tone of the book's narrator made it feel like a close friend was telling the story.