The Spanish verb 'feriar' comes from the Latin word 'feria', which meant 'holiday' or 'festival'. In Spanish, the noun 'feria' evolved to specifically mean 'fair' or 'market'. The verb 'feriar' was then formed by adding the common Spanish verb suffix '-ar' to 'feria', creating a verb meaning 'to trade' or 'to sell at a fair'.

This etymology reflects how holidays and festivals in ancient times were often occasions for trading and commerce, as people would gather together not just to celebrate but also to buy and sell goods. This commercial aspect became emphasized in the Spanish terms.

The Spanish noun 'feria' is directly related to 'feriar' and is actually more common. While 'feriar' means 'to trade at a fair', 'feria' simply means 'fair' or 'market'. You might also encounter 'feria' meaning 'fair' in the sense of a carnival or festival, maintaining some of its original Latin meaning of 'holiday'.

The English word 'fair' (as in 'county fair' or 'trade fair') is actually cognate with Spanish 'feria' and 'feriar', though it came to English through a different route. Both ultimately trace back to the Latin 'feria'. The English word 'feast' and 'festival' are also related, all stemming from the same concept of a holiday or celebration that often involved trading and festivities.