Inherited from Vulgar Latin *pueruncus, from Latin puer (“child”) + diminutive suffix -unculus. In Vulgar Latin, a popular, syncopated variant of the suffix -unculus was *-unclus, hence words like unchi, rărunchi, genunchi, etc. This *puerunclus was hypercorrected by analogy with Latin iuvencus ("calf, bullock"; inherited as junc), resulting in an unusual *pueruncus. While the hypercorrection itself is unusual, unusual hypercorrections in Romanian are not something unthinkable (see how curună evolved into cunună in Romanian by being misperceived as rhotacized).
See also the word trunchi for another example of a word in Romanian being displaced by its diminutive variant.
prunc m (plural prunci)
| singular | plural | ||||
|---|---|---|---|---|---|
| indefinite | definite | indefinite | definite | ||
| nominative-accusative | prunc | pruncul | prunci | pruncii | |
| genitive-dative | prunc | pruncului | prunci | pruncilor | |
| vocative | pruncule | pruncilor | |||