degrammaticalize

verb2

1. •
   To make less grammatical or not grammatical.
2. •
   To cause (an inflectional ending, etc) to undergo degrammaticalization.

   • Using diachronic evidence from Swedish, I will show that inflectional endings may be maintained as “less cumulative” inflections, or even degrammaticalize into a derivational suffix or a clitic. — 2002