Typologically-unexpected overt expletives can be found in a restricted number of non-standard Ibero-Romance null-subject varieties. Historical data suggest that these overt expletives, which in today’s varieties show both discourse-oriented and expletive characteristics, have their origin in 15th century impersonal epistemic constructions. This article argues that it is the expletives’ epistemic origin which gives rise to, and thus explains, their present-day heterogeneous properties, in particular their function as a marker of epistemicity in a number of varieties. Despite undergoing the same mechanisms of change, the variation in modern Ibero-Romance is understood to be a consequence of the different stages and degrees of grammaticalization reached in each variety.