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.
