Definitionbuttocks, because the large smile they crack. See buttocks for synonyms.
2. That smile itself, the large grin between the buttocks.
Synonyms: ass crack; backeye slice; backeye slit; brownsward; bum cleavage; butt-crack; buttocks cleavage; cleft; crack; crease; crevice; foul line; reverse cleavage; split; vertical grin; valley.
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broad in the beam
broaden someone's outlook