What The Moo? SOMEONE HELP ME OUT!!!

Hey guys

Just wondered if anybody knew what the phrase "Butter Wouldn't Melt In Her Mouth" actually means?
I know it means innocence but in what way is butter relevant?
Me and my good friend Ruth were discussing this and we couldn't think of any logical reason

HELP PLEASE!!!

love
XxFeEBeDeExX

I don't know, she doesn't have a hot mouth? like she doesn't mouth off/sware or some thing? Just guessing...