The Spanish word 'pendejo' comes from Latin 'pectiniculus', which meant 'small pubic hair'. This Latin word was formed by combining 'pecten' (meaning 'comb' or 'pubic hair') with the diminutive suffix '-iculus'. Over time, the word evolved both phonetically and semantically in Spanish, where it initially retained its anatomical meaning but later developed into a vulgar insult meaning 'fool' or 'idiot'. This semantic shift from a literal anatomical term to an insult follows a common pattern in the development of vulgar language.

The Spanish word 'peine' (meaning 'comb') is a related word, as it also derives from the Latin 'pecten'. While 'pendejo' took on a vulgar meaning, 'peine' maintained its connection to the original meaning of a grooming tool.

The English word 'pectinate' (meaning 'having teeth like a comb') comes from the same Latin root 'pecten'. Similarly, the scientific term 'pectoral' (relating to the chest) shares this ancestry, as it originally referred to the comb-like arrangement of chest muscles.