The present article examines the grammatical origin of the negator NEG ___ bu – and more particularly its enclitic element bu – which has been created in Moroccan Arabic in the Oujda region, north-eastern Morocco, by contact with Berber (Tarifit language, northern Morocco, see Figure 1; see also Lafkioui (2013).1 As discussed elsewhere (Lafkioui 2013), Moroccan Arabic negation has been subject to certain contact-induced innovation processes, through which the morphological data as well as its syntactic structuring and semantic functioning have been modified by analogy with Berber negation. This study aims at explaining where the negator NEG___ bu originates from by developing mainly two diachronic scenarios. These are (1) the grammaticalization of a verbal form related to iba (or variant) meaning ‘there is no’ and presently occurring in Tuareg Berber and (2) the grammaticalization of the nominal head bu, attested in Berber as well as in Arabic.