The Spanish word 'lagarto' (meaning 'lizard') comes from the Vulgar Latin term '*lacartus', which was a Hispanic variant of the Classical Latin word 'lacertus'. In Classical Latin, 'lacertus' had two interesting meanings: it could refer to a lizard, but it was also used to describe the upper arm muscle. This dual meaning likely came from the way a flexed bicep might resemble the shape or movement of a lizard. As the Latin language evolved in the Iberian Peninsula, the pronunciation shifted from 'lacertus' to '*lacartus' and finally to 'lagarto', with the 'c' softening to a 'g' sound - a common sound change in the development of Spanish from Latin.

A common diminutive form of 'lagarto' is 'lagartija', which refers specifically to a small lizard or gecko. You might also encounter 'lagartera' (a lizard's den) or 'lagartero' (relating to lizards). These words form a family of reptile-related terms that all stem from the same Latin root.

While English 'lizard' is not directly related to 'lagarto', English borrowed the Latin 'lacertus' in scientific terms. For example, the 'lacertus' muscle refers to the biceps brachii muscle of the upper arm, preserving the Latin word's anatomical meaning. The scientific name for the family of typical lizards is also 'Lacertidae', directly borrowed from the same Latin root.