John Cartmell wrote:

Sill comes from Old English syll, which in turn derives from the
proto-Germanic/Indo-European words suljo/swel meaning beam. You find it as
sulle, sille, svill, &c in various early Germanic languages. Never as cill -
except that the OED allows cill as an occasionally used variant of sill.
Occasionally by people who spell it wrong? ;-)

For everybody's reference. It's sill (or cill - but preferably sill). And it
doesn't mean inside or outside - but either.

Sill or cill is a supporting rail as in the bottom rail of a window or
door (threshold). It's also the name of the base beam on which timber
framed houses are built.

But most people also use the term erroneously to refer to the board
that closes the bottm of the window opening internally.

It would be more correctto refer to the outside lip if the window is of
the type that has one as a sill is also a ledge in geology. (Or a seam
with that tendency, I think.)