This paper presents the first comprehensive analysis of the forms and functions of mouth activities in Turkish Sign Language and focuses on the role of Turkish full/reduced mouthings (i.e., /var/, /va/, /vı/) and mouth gestures (i.e., biting lips) in existential constructions with/without the manual existential markers VAR1 and VAR2. The findings indicate that the construction [SIGNnon-existential + mouthing /var/] first gains an existential/possessive function through morphosyntactic development. The phonological structure of the Turkish full mouthing /var/ then undergoes a reduction, resulting in the articulation of /va/ or /vı/. Ultimately, it evolves into the biting lips mouth gesture, representing the initial sound of /var/, 'v', which is a labiodental fricative. The evolving biting lips pattern, derived from the mouthing of /var/, provides a compelling case for re-evaluating the classification of "mouth gesture" in the literature. The preceding observations regarding the development of mouthings to mouth gestures thus lead to a number of theoretical questions concerning the nature of multimodal grammaticalization. In order to address these inquiries, the present study proposes an additional route, referred to as Route III, which builds upon the multimodal constructional development and differs from Wilcox's Route I and II in that it pursues the objective of "gesturalization".