The Spanish word 'cosa' meaning 'thing' comes from the Latin word 'causa', which meant 'cause, reason' or 'legal case'. This semantic shift from 'cause/reason' to the more general meaning of 'thing' is interesting - you can imagine how a 'cause' or 'matter at hand' gradually became generalized to refer to any object or matter, until it became the general word for 'thing' that we see in modern Spanish.

The Spanish word 'causa' is directly related to 'cosa', as it retained the original Latin meaning of 'cause' or 'reason'. You might also encounter 'causar' (to cause) in Spanish, which shares this same Latin root.

English speakers will recognize this etymology in words like 'cause', 'because', and 'causal', which all come from the same Latin root 'causa'. While the Spanish word broadened its meaning to 'thing', the English descendants maintained the original sense of 'reason' or 'that which brings about an effect'.