extensión
ES [ekstenˈsjon] · Spanish for “expanse, area” · looks like English extension
I ran into this one refusing to switch a Spanish Wikipedia article back to English. The page kept saying extensión and I kept quietly reading it as extension, as though the desert had been enlarged at some point or had a wing built onto it. All it meant was the area: the ground the thing covers, edge to edge.
What makes it slippery is that the two are near enough to pass. Meet extensión in a contract and your English instinct about extra time may land perfectly well. Meet it in front of a map and you are quietly reading the wrong word.
Both come from Latin extensio, a stretching out, off the verb extendere - ex plus tendere, to stretch. Latin meant the state of being stretched, and Spanish stayed there: an extensión is how far something reaches, which is why a farm has una extensión de cien hectáreas. English pushed the word towards stretching something further and settled on the added piece - an extension on a house, an extension on a deadline, an extension cord. The English word that genuinely lines up with the Spanish one is extent, grown from the same verb through Old French and so extensión's own twin. Spanish will still say extensión for a phone line or the letters after a filename, so the pair never fully pulls apart. It leans one way and English leans the other, and the lean is where people trip.