Phil, I never heard the expression "Hotter than a witches tit in a brass bra?" However, I do recollect from my college experience many years ago, on very cold days we would use the expression "It's colder than a witch's tit." We must have gone to different schools <g>.
AK
p.s. Apologies for not being PC, so all feminists here, please share the sexist versions you learned in school <g>. (Sarals, you here?)