The Spanish word 'primo' meaning 'cousin' (and sometimes 'first' in certain contexts) comes from the Latin word 'primus' meaning 'first'. The semantic shift from 'first' to 'cousin' likely developed from the concept of first-degree relatives or first-degree relationships within a family tree.

Several common Spanish words share this Latin root, including 'primero' (first), 'primavera' (spring, literally 'first season'), and 'primitivo' (primitive, original). The connection between these words lies in their reference to something that comes first or is original.

English speakers might recognize this root in words like 'prime' (first in importance), 'primary' (first, main), 'primitive' (original, early), and 'primordial' (existing at the beginning). All these words carry the sense of being first or original, maintaining a connection to the Latin 'primus'.