Smart as hell.
Well, that metaphor didnt used to be so ubiquitous. The, "as hell" comparitive metaphor, originated from, "hot as hell" which obvious came to be, since hell is supposed to be on fire, and so anything hot as hell, would be the most extreme level of hot. However, over time, "as hell" just began being used with other terms in the similar way to mean it being the most of the most or just the most super extreme for whatever the qualifying word used preceding it was, such as in this instance, where it is being preceded by "smart." But as far as I am aware, hell is not supposedly known as being an extremely smart place.
"As hell" is an adverb used as an intensifier, meaning 'to a great extent or degree', 'very'.