The generally accepted view, is that English 'sir' and 'sire' are derived from Latin 'senior' meaning 'elder' and they were used as a marker of respect before they were used to denote nobility.

The same applies in French and Spanish which are Romance languages with strong links to Latin.

Not every etymologist will agree (the theory assumes a vulgar latin form of 'senior' with the 'n' dropped, '*seior' which isn't properly atttested in the written record) but it is the generally accepted position, endorsed by the OED as being the best existing etymology.

Incidentally the female equivalents 'Dame' and 'madam' both derive from the latin 'Domina', the female form of 'Dominus' meaning 'master'