Beable is an expression coined by J. S. Bell to describe elements of reality in quantum mechanics, i.e., "what exists out there". It became necessary for physicists to have such a world given that "observables" can, in some interpretations of the theory refer to things that do not exist priorly to their observation, or do not even exist at all.
The term has been criticized by Shimony:
The suffix "able" etymologically refers to a potentiality. An observable of a system is a property which can be observed, even though it may actually be the case that no one has observed it. But Bell's criterion for applying the term "beable" to things is that "they are there" (Ref. 3, p.ll); no potentiality is involved. The term "existent" would have been more accurate than his neologism. I hope that Gresham's law will not be confirmed in the present case.
Gresham's law is that bad money drives out good money, or, here, an inferior terminology drives out a superior one. Gresham's law did apply.
The Urban Dictionary defines it as the noise of a swan.