Mooncalf was a term formerly ascribed to the abortive fetus of a cow or other farm animal, and also occasionally to that of a human.
The term arose from the formerly widespread belief, present in many European folk traditions, that such malformed creatures were the product of the sinister influence of the moon on fetal development.
Modern usage
The term came to be used to also refer to any monstrous or grotesque thing. Shakespeare, for instance, used the term to describe Caliban, the deformed servant of Prospero, in The Tempest.
Mooncalf is used as a derogatory term to indicate someone is a dullard, fool or otherwise not particularly bright or sharp. For example, W. C. Fields in "The Bank Dick" (1940) advises his prospective son-in-law to avoid being a "mooncalf" by buying shares he has been beguiled into believing are worth much more than the proffered price.