The Spanish word that obeys its own rule

«Esdrújula» — the Spanish term for words stressed on the antepenultimate syllable, like «música» or «sábado» — comes from the Italian «sdrucciolo», meaning 'slippery'. Renaissance treatises on metre used it for verses whose endings seem to slide downhill when spoken. Spanish adopted it and, as it always does with impossible initial S-clusters, propped it up with an E: from «spiritus», espíritu; from «sdrucciolo», esdrújula.

The lovely part is that the word practises what it preaches: «esdrújula» is itself esdrújula. Linguists call such words autological, and Spanish has a small festival of them: «llana» (stressed on the penultimate) is llana, «breve» (short) is brief, «pentasílaba» (five-syllabled) has exactly five syllables.

There is one hidden betrayal, though: «aguda», the word for words stressed on the final syllable, is not aguda. It's llana. Nobody said language had to be fair.