Sidi is a masculine title of respect, meaning "my master" in Western Arabic language and Egyptian Arabic (sayyid in other dialects) equivalent to modern popular usage of the English Mr.. It is also used in other Arabic dialects, such as Eastern Arabic, as well as by Muslims of the Indian subcontinent in the Urdu language; however, in these latter two cases, "Sidi" does not have so much currency as titles like sayed, janab or saheb.
Occasionally a respected member of a Muslim society will be given the title Sidi by default in recognition of upright standing and wisdom. This especially applies to marabouts, hence the term appears in places and mosques named after one.
In the Maghrebi kingdom of Morocco:
- Sidi ('Lord'): the title used as a substitute for Mulay by those male members of the ruling Alawi dynasty sharing the first name of the Muslim Prophet, Muhammad
- Smiyet Sidi: a style usually reserved for the Sovereign or the Heir Apparent, loosely, but imperfectly, translated as 'His Lordship' or 'Monseigneur'
In neighbouring Tunisian Arabic, it has evolved into two separate variants: "Si" means Mister or Sir, while "Sidi" means "Master" or "Saint", depending on the context. For example older conservative Tunisian women call their husbands "Sidi", not by first name; older conservative Tunisians (both men and women) call their older brothers "Sidi", especially when there is a large age difference between them. There are two female equivalents of "Sidi": "Lella", when used in a social context, and "Sayyeda" when speaking about female saints, for example "(Es)-Sayyeda El-Manoubiyya".
In the Hijaz region of Saudi Arabia, the word sidi is used as an honorific when addressing an older brother by many of the more traditional families.
The term was also used by Arabized Spanish, Portuguese and Berbers in Mozarabic-speaking Moorish Spain (cf. El Cid).