IPA: /ˈsmuːðən/
KK: /ˈsmuːðən/
To make something smooth or even, removing bumps or roughness.
She used a roller to smoothen the surface of the wall before painting it.
Past: smoothened
Past Participle: smoothened
Smoothen → It is formed from "smooth" (from Old English *smoð*, meaning even or level) and "-en" (a suffix used to form verbs indicating to cause to become). The word "smoothen" means to cause something to become smooth or even.
Think of making something 'even' or 'level' — that's what smoothen means, to make something smooth.