This paper examines the evidence from Middle Welsh for the emergence of the Modern Welsh marker of clausal negation ddim. It considers cases where the indefinite pronoun dim ‘anything’ appears not to be an argument of its verb, and therefore can be considered to be a ‘pseudoargument’ with an adverbial function ‘at all’. Isolated examples of this use can be found from the thirteenth century onwards, with robust attestation in a number of texts from the fourteenth century. It is argued that the pattern of attestation is consistent with pseudoargument dim being a late-thirteenth-century innovation, found only in texts first committed to writing from that time onwards. The primary factor in its emergence is argued to be the potential ambiguity in the analysis of optionally transitive verbs. Syntactic differences between Middle Welsh pseudoargument dim and the present-day Welsh clausal negation marker suggest that the former is not the direct ancestor of the later, but rather Middle Welsh pseudoargument dim instead survives in semi-fossilised form as a sentence-final adverbial today.