a foolish or absentminded person
About the word:
The original mooncalf was a false pregnancy, a growth in the womb supposedly influenced by a bad moon. Mooncalf then grew a sense outside the womb: simpleton. It also morphed into a literary word for a deformed monster. For instance, in Shakespeare's The Tempest, Stephano entreats Caliban, "Mooncalf, speak once in your life, if thou beest a good mooncalf."