The Spanish verb 'ser' (to be) has an interesting etymology that combines two Latin verbs: 'sedere' meaning 'to sit' and 'esse' meaning 'to be'. The Latin verb 'esse' was irregular and evolved into a more regularized form 'essere' in Late Latin. In Old Spanish, these forms merged into 'seer', which was influenced by both the meaning of 'to be' from 'essere' and possibly the concept of 'sitting/being in a state' from 'sedere'. This Old Spanish 'seer' eventually simplified to the modern Spanish 'ser'.

This evolution helps explain why 'ser' is used for permanent or inherent states of being in Spanish, as it partially derives from a word meaning 'to sit' - suggesting a permanent or settled state.

The Spanish verb 'sentar' (to sit) and its reflexive form 'sentarse' are distant relatives of 'ser' through the Latin 'sedere'. While 'ser' evolved to express existence or being, 'sentar/sentarse' retained the original physical meaning of sitting. The word 'sede' (seat, headquarters) is another Spanish relative that preserved the connection to sitting.

Several English words are etymologically related to 'ser' through its Latin ancestors. From 'sedere' (to sit), we get English words like 'sedentary' (involving much sitting), 'sedate' (calm, quiet - like someone sitting still), and 'session' (a sitting of a court or other group). The 'esse' branch of the family gives us English words like 'essence' (fundamental nature of something), 'essential' (necessary, fundamental), and 'entity' (something that exists).