The writings of the Cairo Geniza (10th through 13th centuries) show that derisive nicknames served as family names even in official documents handed over to the parties concerned. After a short general survey of the use of by-names (alqab) in Arabic and Hebrew literatures, the development of a nickname into a family name is demonstrated by an example from the ethno-linguistic studies of the author among the Jews of Yemen. The main part of the paper discusses Arabic nicknames from the Geniza documents of which it can be proved that they were used as family names.