IPA: /ˈhɑːrʃən/
KK: /ˈhɑrʃən/
To make something rough, severe, or unpleasant in quality or tone.
The criticism served to harshen the atmosphere in the room.
Past: harshened
Past Participle: harshened
Harshen is formed from "harsh" (from Old English *heard*, meaning hard or severe) and "-en" (a suffix used to form verbs indicating to make or become). The word "harshen" means to make something harsh or to become harsh.
Think of something becoming 'hard' or 'severe' — that's what 'harshen' means, to make something harsh.