The Spanish word 'pelo' meaning 'hair' comes directly from the Latin word 'pilus', which also meant 'hair'. This is a straightforward etymology where both the meaning and form have remained quite stable over time, with just the ending changing from '-us' to '-o' as was common when Latin words evolved into Spanish.

Some related Spanish words include 'peludo' (hairy), 'peluquería' (hair salon), and 'pelaje' (fur, coat of an animal). These words are all built from the root 'pel-' referring to hair.

While English took a different path for its basic word for 'hair', we do have some words from the same Latin root. 'Depilatory' (a substance used to remove hair) comes from Latin 'depilare' ('de-' meaning 'remove' + 'pilus' meaning 'hair'). The word 'pile' when referring to the fuzzy surface of carpet or velvet also comes from Latin 'pilus'.