Butter Wouldn’t Melt in (Someone’s) Mouth

What does the idiom “Butter Wouldn’t Melt in (Someone’s) Mouth” mean?

Definition:

This person is cool in manner, prim and proper

For example:

“Jones has exceptional presence of mind on the soccer field. He always looks like butter wouldn’t melt in his mouth.”

Notes:

This idiom is less common in the USA, but is occasionally used.