'yerba' originates from Spanish, specifically the word 'hierba', where the Latin root 'herba' meant 'grass, herb'.

'hierba' (Old Spanish) derived from Latin 'herba'; over time the initial 'h' became silent in Spanish and regional pronunciation/spelling led to forms like 'yerba' in some dialects and in much of Latin America.

Initially it meant 'grass' or 'herb' in Latin and Old Spanish; that basic meaning has largely remained, though in modern English 'yerba' is frequently used to refer to particular South American plants (for example, yerba mate).