The Spanish word 'bollo' meaning 'bun' or 'roll' comes from the Latin word 'bulla' meaning 'bubble' or 'ball'. The evolution went through an intermediate form 'bolla' meaning 'round bread' in early Spanish. This etymology makes intuitive sense since bread rolls and buns typically have a rounded, ball-like shape similar to bubbles.

A related Spanish word is 'bola' meaning 'ball', which comes from the same Latin root 'bulla'. You can see how both 'bollo' and 'bola' retained the basic concept of roundness from their Latin ancestor, with 'bollo' specifically applying to rounded bread products.

The English word 'bull' (as in a papal bull) is related, coming from the same Latin word 'bulla'. In ancient Rome, a 'bulla' was a round seal attached to documents, and this is where we get the term 'papal bull' - an official document from the Pope. The round seal shape connects it to the same etymology as Spanish 'bollo'. Another related English word is 'bulletin', which originally referred to a document with an official seal.