The Spanish verb 'firmar' (to sign) comes from the Latin verb 'firmare', meaning 'to make firm, strengthen, or confirm'. This Latin verb was derived from the adjective 'firmus', meaning 'firm, strong, or stable'. The semantic evolution from 'to make firm' to 'to sign' makes sense when you consider that signing something makes it official or confirms it - essentially making it 'firm' in a legal or official sense.

Some related Spanish words include 'firme' (firm, steady), which directly preserves the meaning of the Latin 'firmus', and 'firmeza' (firmness, steadiness). The connection between these words and 'firmar' shows how the concept of making something firm or official evolved into the act of signing.

English speakers can easily connect 'firmar' to words like 'firm', 'confirm', and 'affirm', which all come from the same Latin root 'firmus'. Just as 'confirm' means to make something definite or valid (often through a signature!), and 'affirm' means to state something as valid or true, 'firmar' represents the act of making something official through signing.