The Spanish word 'mil' meaning 'thousand' comes directly from the Latin word 'mille', which also meant 'thousand'. This is a straightforward evolution where the Latin word was simplified in Spanish by dropping the final '-le' syllable.

Some common Spanish words derived from the same root include 'millón' (million), 'milímetro' (millimeter), and 'milenio' (millennium). All these words carry the basic meaning of 'thousand' in their formation - for example, a millennium is a period of a thousand years.

Many English words share this Latin root, making them cognates with the Spanish 'mil'. Some common examples include 'mile' (originally based on a thousand paces), 'million', 'millennium', and 'millimeter'. The prefix 'milli-' in English, meaning one-thousandth (as in milliliter or millisecond), also comes from this same Latin root.