The Spanish word 'asedio' meaning 'siege' comes from the Spanish verb 'asediar' ('to besiege'), which in turn derives from the Latin verb 'obsidiāri' meaning 'to besiege'. This Latin verb was derived from another Latin verb 'obsidēre', also meaning 'to besiege'. The change from Latin 'ob-' to Spanish 'a-' at the beginning of the word is a common phonetic development in Spanish, while the '-sidēre' part evolved to '-sediar' following regular sound changes between Latin and Spanish.

A related Spanish word is 'sede' meaning 'seat' or 'headquarters', which comes from the same Latin root 'sedēre' meaning 'to sit'. The connection makes sense when you think about a siege as the act of 'sitting around' or 'sitting before' a place until it surrenders.

English speakers might recognize a connection to the word 'obsidian' (the volcanic glass), which, while not directly related in meaning, comes from the same Latin root 'obsidēre'. The stone was supposedly named by Romans who found it 'sitting around' in various places. Another related English word is 'sedentary', meaning 'characterized by sitting', which shares the same Indo-European root relating to sitting.