To smile broadly, especially in a way that shows the teeth, often expressing amusement, pleasure, or sometimes mischief.
From Old English 'grennian' meaning to show the teeth in pain or anger, related to 'grin' meaning snare. The positive connotation of pleasure or amusement developed later, though traces of the original meaning persist in phrases like 'grin and bear it.'
The evolution of 'grin' from expressing pain to expressing joy illustrates how facial expressions can be ambiguous - both emotions involve showing teeth. Interestingly, the phrase 'grin and bear it' preserves the word's original connection to enduring discomfort.
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