Sure As Eggs Meaning

(idiomatic, colloquial) surely; doubtlessly

Example: 1991, Knut Faldbakken, Insect summer
  "Pity for you though, all the same, you could have tried your luck there — she'll open her legs for grammar-school boys, sure as eggs."
"But if she's not much to look at anyway," I said, thereby saving both him and me...