Sill
Old English syll, sylle ‘horizontal beam forming a foundation’, of Germanic origin; related to German Schwelle ‘threshold’.
wiktionary
From Middle English sille, selle, sülle, from Old English syll, syl(“sill, threshold, foundation, base, basis”), from Proto-Germanic *sulī(“bar, sill”), from Proto-Indo-European *sel-, *swel-(“beam, board, frame, threshold”). Cognate with Scots sil, sill(“balk, beam, floor, sill”), Dutch zulle(“sill”), Low German Sull, Sülle(“threshold, ramp, sill”), Danish syld(“base of a framework building”), Swedish syll(“joist, cross-tie”), Norwegian syll, Icelandic syll, sylla(“sill”). Related also to German Schwelle ( > Danish svelle), Old Norse svill, Latin silva(“wood, forest”), Ancient Greek ὕλη(húlē).
Compare sile.
Compare thill.
etymonline
sill (n.)
Old English syll "beam, threshold, large timber serving as a foundation of a wall," from Proto-Germanic *suljo (source also of 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" (source also of Greek selma "beam"). Meaning "lower horizontal part of a window opening" is recorded from early 15c.
