smoothen

verb

make or become smooth

Wiktionary

Pronunciation: /ˈsmuːðən/

verb

Etymology: Etymology tree Old English smōþ Middle English smothe Proto-West Germanic *smanþī Old English smēþe Middle English smethe English smooth English -en English smoothen From smooth + -en.

To make smooth.
“Men were smoothening the thatch, which ran from the top of the roof to the ground covering the sides and at the back of the building.”
To become smooth.