The Spanish word 'bajo' meaning 'low' or 'short' comes from the Vulgar Latin word 'bassius', which had the same meaning. This Vulgar Latin term developed from Classical Latin 'bassus', which meant 'fat and short'. Over time, the meaning generalized from specifically describing short and stout things to describing anything that was low or short in height or position. The sound changes from Latin 'bassus' to Spanish 'bajo' followed a common pattern where the Latin '-ss-' became '-j-' in Spanish, and the final '-us' was simplified to '-o'.

Several common Spanish words are related to 'bajo', including: 'abajo' (down, downstairs), 'debajo' (underneath), and 'bajada' (descent, slope). The adjective 'bajo' is also used as a noun meaning 'bass' in music, and appears in many compound words like 'bajorrelieve' (bas-relief) where it maintains its meaning of 'low'.

While English 'base' might look similar to Spanish 'bajo', they actually have different etymological origins. However, English has borrowed some words that are related to 'bajo' through French, such as 'bass' (as in the low musical pitch) and 'bas-relief' (a type of sculpture where figures project slightly from a flat background). These came from the same Latin root 'bassus' that gave Spanish 'bajo'.