Sheng is Kenya's rapidly growing urban vernacular. Spyropoulos (1987), quoting Mkangi (1985), states that Sheng arose in pre-independent Kenya as indigenous peoples from different ethnic and language backgrounds started converging on Nairobi as migrant workers with the advent of the colonial economy. In terms of structure, Sheng appears to fuse grammatical aspects mainly from English and Swahili, but also incorporates elements from the forty-plus Kenyan languages spoken alongside Swahili and English. How has Sheng grown amid these 'established' languages, especially the co-official languages of Swahili and English? This paper traces and discusses various claims about the origins and characterizations of this code and argues that in terms of form and function, Sheng is a hybrid language defying clear classification into the traditional language contact outcomes (Thomason 1997).