The Spanish word 'pero' (meaning 'but') comes from the Latin phrase 'per hoc' meaning 'therefore'. This Latin phrase was formed by combining two words: 'per' meaning 'through' or 'by means of', and 'hoc' meaning 'this'. Over time, as Latin evolved into Spanish, 'per hoc' underwent phonetic changes and semantic shift, becoming 'pero' and changing its meaning from 'therefore' to the contrasting conjunction 'but'.

This kind of semantic change, where a word shifts from expressing a logical conclusion ('therefore') to expressing contrast ('but'), shows how language meaning can evolve in unexpected ways over centuries of use.

While 'pero' itself is one of the most basic and common Spanish conjunctions, the Latin 'per' element survives in many Spanish words with the sense of 'through' or 'by means of', such as 'permitir' (to permit/allow) and 'perfecto' (perfect).

English has many words derived from the Latin 'per-' prefix, including 'perfect' (thoroughly made), 'permit' (to let something through), and 'persist' (to thoroughly continue). The Latin demonstrative 'hoc' is related to English words like 'here' and 'hence', though these connections are more distant.