This paper aims to make a textual investigation of weak adjectives manifested in the Old English epic Beowulf and provide the minimalist-based morphosyntactic accounts for their distributions. It furthermore examines how weak adjectives are associated with diachronic changes in nominal definiteness in terms of grammaticalization. The suggestion is made that weak adjective paradigm in Beowulf represents nominal definiteness in early Old English. It frequently constitutes a functional head for definiteness within determinerless NPs but sometimes shows signs of reanalysis into the affix. The coincidence of the weak adjective and the demonstrative is understood as evidence for the adoption of the demonstrative for a definiteness marker through renewal. Then, the new cycle on nominative definiteness begins, with the demonstrative becoming subject to unidirectional progression in the cline of grammaticalization.