Mooncalf is a term, now vanishingly rare, used to Describe a simpleton, but in the century or so that it was used, it has had several meanings.

In the mid sixteenth century calf was used both as a term of endearment and pejoratively to describe a person. Mooncalf first appeared, in a thesaurus from 1565, to describe the phenomenon of a false pregnancy, one that had been created under the baleful influence of the moon. Ralph Tyler, in Five Godlie Sermons (1602), used it in that context; “…in that they haue not a charge of their bodies but the cure and care of their soules and as Midwiues to discerne the moone calfe from the perfect fruite of weomen so Preachers should not bring forth moone calues”.  

By the time Shakespeare used it in The Tempest, it had the meaning of a deformed monster, Stephano entreating Caliban, “mooncalf, speak once in your life, if thou be’est a good mooncalf”. Presumably, it extended the concept of a malevolent moon that had moved on from producing phantom foetuses to foetuses that were malformed.

Less than half a century later, John Booker was using it in A Rope for a Parret (1644) to denote a simpleton, someone who mooned their time away daydreaming; “Thou Mercury very Ridiculous, Thou Bloxford flye,Thou Moon calfe, born that very hour, on that very dismall fifth day of the moneth (you remember the Gun-powder Treason) when thy brother G. Faux was caught with a dark Lanthorne….”.

This is the meaning that stuck until it fell into obscurity.