Growing up, my father would often use the phrase “grinning like a Cheshire Cat”, when he wanted to make the, not so flattering, point about a stupid, dim-witted  and otherwise display of adulation by someone, for someone else.

The display of adulation part of his use the phrase is the part I have long held as the most applicable definition when I look at and think about their grin, as one like that of the Cheshire Cat.