This study investigates weak drop, a deletion process in which a two-handed sign is realized as one-handed, in Shanghai Sign Language, centering on comparing deaf signers and hearing non-signers. I propose that knowledge of the sign language grammar mediates the weak drop acceptability judgment of a subset of signs (unbalanced signs) and the weak drop production of all signs. This fine-grained analysis of nuances between signers and non-signers helps shed light on areas where a linguistic system in the visual-manual modality comes into play.