Old English syll "beam, threshold, large timber serving as a foundation of a wall," from Proto-Germanic *suljo (cognates: Old Norse svill, Swedish syll, Danish syld "framework of a building," Middle Low German sull, Old High German swelli, German Schwelle "sill"), perhaps from PIE root *swel- (3) "post, board" (cognates: Greek selma "beam"). Meaning "lower horizontal part of a window opening" is recorded from early 15c.