Ellipsis of if it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

(US, slang, vulgar) Extremely tight (in various senses: secure, stingy, etc.).

plural of duck's-bill limpet

A limpet of the genus Scutus.

An Arctic sea bird, Clangula hyemalis.

(programming) The practice in which a programmer attempts to solve a bug by explaining their code to an inanimate object, such as a rubber duck.

plural of duck-billed cat

(US, dialect, chiefly Mississippi, dated) The paddlefish.

(simile, idiomatic) Alternative form of like a duck takes to water

A belief in a supernatural or paranormal phenomenon that people continue to hold, in spite of it being debunked.