The Spanish word 'bolsillo' meaning 'pocket' has an interesting journey from ancient Greek through Latin. It starts with the Greek word 'byrsa' (βύρσα) meaning 'leather' or 'hide'. This word was borrowed into Latin as 'bursa', where it came to mean specifically a 'leather bag'. The Latin word evolved into Spanish 'bolsa' meaning 'bag', and then combined with the Spanish diminutive suffix '-illo' to form 'bolsillo' - literally meaning 'little bag', which came to refer specifically to a pocket (which, when you think about it, is essentially a small bag sewn into clothing).

The most obvious related Spanish word is 'bolsa', which means 'bag' or 'purse'. This is the base word from which 'bolsillo' is derived. While 'bolsa' refers to any kind of bag or sack, 'bolsillo' specifically evolved to mean the small bag-like compartment in clothing that we call a pocket.

The English word 'purse' is actually a cousin of 'bolsillo', as it also comes from the same Greek root 'byrsa' through Latin 'bursa'. While 'bolsillo' developed to mean a pocket sewn into clothing, 'purse' maintained its meaning as a separate container for money and personal items. The word 'bourse', meaning a stock exchange (especially in European contexts), is another English relative - it got this meaning because merchants would historically gather at a specific building to trade, and that building was named after the Latin 'bursa' due to its association with money and trading.