O.E. syll "beam, large timber serving as a foundation of a wall," from P.Gmc. *suljo (cf. O.N. svill "framework of a building," M.L.G. sull, O.H.G. swelli, Ger. Schwelle "sill"), perhaps from PIE base *swel- "post, board" (cf. Gk. selma "beam"). Meaning "lower horizontal part of a window opening" is recorded from 1428.
[sil] 