You cannot hide an eel in a sack.

The image suggests that an eel’s movement would betray its presence even when the animal is covered. Putting it in a sack would not make it truly hidden.

Figuratively, the proverb describes a fact or situation that cannot be kept secret because signs of it remain noticeable. An attempt to conceal it may be futile.

Use it when someone tries to hide something that continues to leave visible clues. It suits a situation where the attempted concealment is less convincing than the person responsible believes.