Sello (Spanish for “stamp”) is a cousin of the English seal — not the animal, but the, umm, stamp that is put onto official documents.
Bot come from the Latin sigillum meaning, “small, engraved picture” because a stamp or seal really was just a small, engraved picture.
The ‑gl- sound of the original sigillum vanished into English so that the English word seal is left with just the vowels around it (e, a) (in English) while in Spanish, the ‑gl- evolved into the similar ‑ll- sound. This is in the same class of evolutions as pl- to ll- (plenty, lleno), fl- to ll- (flare, llamar), and cl- to ll- (call, llamar) as well, although less common than those.
