The term derives from the once widespread superstition, present in many European folk traditions, that such malformed creatures were the product of the sinister influence of the Moon on fetal development. The earliest recorded use of the term was in 1565, referring to a human false pregnancy.

What does Mooncalf mean in slang?

: a foolish or absentminded person : simpleton he was a helpless mooncalf of a man, poorly equipped for a grueling life— Peter Schjeldahl.

What is a Mooncalf Harry Potter?

In modern terminology, a "mooncalf" is a foolish person. The term originally meant a deformed calf born under the malign influence of the full moon.