Triangle

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late Middle English: from Old French triangle or Latin triangulum, neuter of triangulus ‘three-cornered’ (see tri-, angle1).

wiktionary

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From Middle English triangle, from Old French triangle, from Latin triangulum, noun use of adjective triangulus(“three-cornered, having three angles”), from trēs(“three”) + angulus(“corner, angle”).

etymonline

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triangle (n.)

late 14c., from Old French triangle (13c.), from Latin triangulum "triangle," noun use of neuter of adjective triangulus "three-cornered, having three angles," from tri- "three" (see tri-) + angulus "corner, angle" (see angle (n.)).