The Spanish word 'merino' comes from the Latin word 'maiorinus', meaning 'of greater size or importance'. This Latin term evolved both in form and meaning as it entered Spanish. The phonetic shift from 'maiorinus' to 'merino' involved the simplification of the diphthong 'ai' to 'e' and the loss of the Latin ending. In Spanish, the word came to specifically refer to the merino sheep breed and its prized wool, likely because these sheep were considered especially important or superior to other breeds due to their fine wool quality.

A related Spanish word is 'mayor', which also comes from the Latin root meaning 'greater' or 'larger'. While 'merino' specialized to refer to the sheep breed, 'mayor' retained the meaning of being greater in size or importance, as in a mayor of a city ('alcalde mayor') or in comparative expressions.

The English word 'major' is a cousin to 'merino', as both ultimately derive from the same Latin root meaning 'greater'. While 'major' maintained the sense of importance or greater size (as in 'major issue' or 'major league'), 'merino' specialized to refer to the wool and sheep breed. English also borrowed the word 'merino' directly from Spanish to refer to the same sheep breed and wool type.