Other forms: windowsills
A windowsill is the little ledge at the bottom of a window. Where the plants sit and the cat watches the road.
The outside version throws rainwater clear of the wall, which is its actual purpose, and the inside version has become a shelf by accident. Deep ones in old houses with thick walls make a seat. The word is written as one word or two, and sill alone is what a builder says. Cats, herbs and unopened post are the three things most reliably found on one.
1nounwindowsillThe ledge at the bottom of a window.
“There were three pots of basil on the windowsill.”
Also accepted in games: sill, window sill
From window + sill.
Etymology from Wiktionary, CC BY-SA