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Old English grennian "show the teeth" (in pain or anger), common Germanic (cf. Old Norse grenja "to howl," grina "to grin;" Dutch grienen "to whine;" German greinen "to cry"), from PIE root *ghrei- "be open." Sense of "bare the teeth in a broad smile" is late 15c., perhaps via the notion of "forced or unnatural smile." Related: Grinned; grinning.
1630s, from grin (v.).