simper like a frumety-kettle, v. — Green’s Dictionary of Slang

simper like a frumety-kettle v.

also simper like a furmity-kettle

[15C–18C SE simper, to simmer + frumenty, a dish made of hulled wheat boiled in milk and seasoned with cinnamon, sugar etc.]

to smile, to look cheerful.

1738		Swift Polite Conversation 22: She simpers like a Firmity Kettle.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Simper. To smile, to simper like a furmity kettle.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.