The Spanish verb 'pensar' (to think) comes from the Latin verb 'pensare', which meant 'to weigh carefully' or 'to ponder'. This Latin word was itself derived from the simpler Latin verb 'pendere', meaning 'to hang' or 'to weigh'. This etymology reveals an interesting metaphorical development: the physical act of weighing or measuring something carefully evolved into the mental act of carefully considering or thinking about something.

Several common Spanish words share this root, including 'peso' (weight), 'pesar' (to weigh, also to regret), and 'pesado' (heavy). The connection between physical weight and mental activity is preserved across these related words, with 'pesar' showing how something weighing on one's mind came to express regret or sorrow.

English speakers might recognize this etymology in words like 'pensive' (thoughtfully or sadly contemplative), 'pendant' (a hanging ornament), and 'pendulum' (a hanging weight that swings back and forth). The word 'pound' (both the weight measurement and the currency) also comes from the same Latin root 'pendere', as it originally referred to weight. Even the word 'expensive' is related, as it literally meant 'weighed out' in money.