This study empirically investigates functional and social variation in the use of TAYYIB (‘okay, well, right’) in light of grammaticalisation. Thirty naturally occurring conversations of 60 Najdi Arabic speakers were recorded in Riyadh, Saudi Arabia. The analysis demonstrates that TAYYIB has two realisations: full Tayyib [tˁajjib] and reduced Tab [tˁab]. Drawing on the conversation analytic approach within a variationist framework, TAYYIB was used to perform multiple discourse-pragmatic functions: interpersonal, textual and interpersonal–textual. The statistical analysis reveals that variant choice is significantly conditioned by the pragmatic functions. While Tayyib is employed to perform all three functions, Tab is only used for textual and interpersonal–textual meanings. As for social factors, Tab is significantly more likely to be used by younger speakers than adults and also more likely to be used by females than males. This can be interpreted as an indication of ongoing change driven by young people, primarily females, towards the greater use of the innovative Tab. Given the evidence of linguistic change in Tab including semantic bleaching, pragmatic strengthening and phonological reduction, the study suggests that Tab has undergone advanced grammaticalisation.