Fig.: it is said about someone who looks innocent, but is actually full of mischief

Angela has this innocent look as though butter wouldn't melt in her mouth, but we know better and we know we cannot trust her

Angela ha un aspetto innocente da perfetta santarellina, ma noi sappiamo bene che non è così e sappiamo di non poterci fidare

Isabelle knew she had scratched her father's car against a low side wall, but with a look as though butter wouldn't melt, she blamed some stranger in the parking lot

Isabella sapeva di avere graffiato la macchina del padre contro un muricciolo, ma lei, con aria da santarellina, ha dato la colpa a uno sconosciuto nel parcheggio

Already a proverb in the work The Proverbs and Epigrams of John Heywood (1562), this metaphoric expression may refer to supposedly virtuous people, who are thought to be so cold and unfeeling that not even butter would melt in their mouth