degrammaticalize

To make less grammatical or not grammatical.

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.

[…] try to provide empirical evidence for the thesis that inflectional affixes can degrammaticalize into a derivational suffix or a clitic.