Sidi is a masculine title of respect, meaning "my master" in Darija 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:
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", a word of Amazigh origin, 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.