filthen

From filth +‎ -en, modelled after healthen, strengthen, lengthen, etc.

filthen (third-person singular simple present filthens, present participle filthening, simple past and past participle filthened)

1. (rare, nonstandard, transitive) To cover with filth; besmut; besmirch; make filthy.
   • 2016, Jennifer Haigh, Heat and Light:

     Rock oil was considered, then, a local nuisance, a malodorous black gunk that floated like a rumor down the creek, filthening whatever it touched: a farmer's overalls, a cow's hide, a child's shoes.