Kitāb al-ʿAyn (Arabic: كتاب العين) is the first Arabic language dictionary and one of the earliest known dictionaries of any language. It was compiled in the eighth century by al-Khalil ibn Ahmad al-Farahidi. The letter ayn (ع) of the dictionary's title is regarded as phonetically the deepest letter in the Arabic alphabet. In addition the word ayn carries the sense of 'a water source in the desert'.
Source: Wikipedia (English) — «Kitab al-'Ayn» — https://en.wikipedia.org/wiki/Kitab_al-%27Ayn
