The Spanish word 'peso' comes from the Latin word 'pensum', meaning 'weight' or 'something weighed'. 'Pensum' itself derives from the Latin verb 'pendere', which means 'to hang'. This etymology reflects how early weighing was often done using hanging scales. The word 'peso' retained its meaning of 'weight' in Spanish, but also evolved to become the name of various Spanish and Latin American currencies, reflecting the historical practice of measuring the value of coins by their weight in precious metals.

Several common Spanish words share this Latin root, including 'pesar' (to weigh, to grieve), 'pesado' (heavy), and 'pesa' (weight, as in a dumbbell). The connection between physical weight and emotional burden can be seen in the expression 'me pesa' (it weighs on me), used to express regret or sorrow.

English has several words that come from the same Latin root 'pendere', including 'pendant' (a hanging ornament), 'pendulum' (a hanging weight that swings), 'suspend' (to hang), and 'pensive' (weighing thoughts in one's mind). The word 'pension' also shares this root, originally referring to a payment that was 'weighed out'.