This paper presents a diachronic account for the emergence of the expletive það in Icelandic impersonal constructions. Using data from the IcePaHC corpus (Wallenberg et al. 2011), I show that a cataphorically referential það functions as a topic position placeholder in Old Icelandic (1150-1350) in impersonals with a clausal argument. The corpus findings indicate that það spread from this early cataphoric context to impersonals which lack the clausal argument, with ‘say-type predicates’ acting as a bridging context. Strikingly, this coincides with another change whereby cataphoric það becomes increasingly restricted to the topic position in constructions with a clausal subject. I interpret this as evidence that cataphoric það is losing its subject status in such contexts and becoming a topic position placeholder, in line with its function in impersonals. This sheds light on the mixed status of cataphoric það in modern Icelandic and challenges the ‘Prefinite First Hypothesis’ for the diachrony of Germanic expletives (e.g. Faarlund 1990).