It has been said that the origin of this name (the Sufi name) is the word safevi, which is derived from safāʾ (purity) or muṣāfāt (mutual purification). Pronouncing this was found to be difficult on the tongue, and the word was thus expressed as Sufi. (The true knowledge of this) belongs to Allah.
I once heard one of our shaykhs say: The word Sufi (is not derived from anything else, but) comes precisely from ṣūfī. (Nabhānī, II, 343)
The word Sufi consists of four letters: ṣād, wāw, fāʾ, and yāʾ. Ṣād stands for his sabr (patient endurance)...