The Spanish word 'peligro' (meaning 'danger') comes from the Old Spanish 'periglo', which in turn derives from the Latin word 'periculum', meaning 'danger, risk, or trial'. The evolution from Latin to modern Spanish shows a typical pattern of sound changes, where the medial '-cu-' sound in 'periculum' first simplified to '-g-' in Old Spanish 'periglo', and then underwent metathesis (switching of sounds) to become modern Spanish 'peligro'.

Some related Spanish words include 'peligroso' (dangerous), which is a common adjective form, and 'peligrosidad' (dangerousness). These words maintain the same root while adding different suffixes to create related meanings.

English speakers might recognize the connection to words like 'peril' and 'perilous', which come from the same Latin root 'periculum'. The English words maintained a spelling and pronunciation closer to the original Latin, while the Spanish version underwent more significant sound changes. Understanding this connection can help English speakers remember that 'peligro' means 'danger' or 'peril'.