снаружи мило, а внутри гнило

• СНАРУЖИ МИЛО, А ВНУТРИ ГНИЛО

[saying]

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⇒ what is attractive on the outside can actually be fundamentally bad, worthless etc (used to emphasize that one should not judge people and things by their outward appearances):

- ≈ fair without but foul within;

- a prize apple can have a worm inside.