Etymology
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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.
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cill 
alternative spelling of sill.
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windowsill (n.)
also window-sill, 1703, from window (n.) + sill (n.).
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mudsill (n.)

1680s, "foundation beam of a dam, railroad, house, or other structure," from mud + sill. The word entered U.S. political history in a figurative sense in a speech by James M. Hammond (1807-1864) of South Carolina, March 4, 1858, in the U.S. Senate, alluding to the necessary lowest class of society. In the speech he also took the ground that Northern white laborers were slaves in fact, if not in name. The term subsequently was embraced by Northern workers in the pre-Civil War sectional rivalry.

In all social systems there must be a class to do the menial duties, to perform the drudgery of life. This is a class requiring but a low order of intellect and but little skill. Its requisites are vigor, docility, fidelity. Such a class you must have, or you would not have that other class which leads progress, civilization, and refinement. It constitutes the very mud-sill of society and of political government; and you might as well attempt to build a house in the air as to build either the one or the other except on this mud-sill. [Hammond]
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liminal (adj.)
"of or pertaining to a threshold," 1870, from Latin limen "threshold, cross-piece, sill" (see limit (n.)) + -al (1). Related: Liminality.
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sublime (adj.)

1580s, "expressing lofty ideas in an elevated manner," from French sublime (15c.), or directly from Latin sublimis "uplifted, high, borne aloft, lofty, exalted, eminent, distinguished," possibly originally "sloping up to the lintel," from sub "up to" (see sub-) + limen "lintel, threshold, sill" (see limit (n.)). The sublime (n.) "the sublime part of anything, that which is stately or imposing" is from 1670s. For Sublime Porte, former title of the Ottoman government, see Porte.

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keelson (n.)

also kelson, "line of jointed timbers in a ship laid on the middle of the floor-timbers over the keel, binding the floor-timbers to the keel," 1620s, altered (by influence of keel (n.)) from Middle English kelsyng (late 13c.), which probably is of Scandinavian origin (compare Swedish kölsvin, Danish and Norwegian kjølsvin), from a compound of words such as Old Norse kjölr (see keel (n.)) + swin "swine," which was used of timber (see swine), or perhaps the second element is a folk-etymology alteration of another word (such as Norwegian svill "sill"). Or else the whole is from a similarly formed Low German source (kielschwin).

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silliness (n.)
"foolishness," c. 1600, from silly + -ness; a reformation of seeliness, from Old English saelignes "happiness, (good) fortune, occurrence."
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sillabub (n.)
also sillibub, syllabub, etc., 1530s, of unknown origin. Drink or dish of milk and wine or cider, often sweetened. Figurative sense of "floridly vapid prose" is from 1706.
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silly (adj.)

Old English gesælig "happy, fortuitous, prosperous" (related to sæl "happiness"), from Proto-Germanic *sæligas (source also of Old Norse sæll "happy," Old Saxon salig, Middle Dutch salich, Old High German salig, German selig "blessed, happy, blissful," Gothic sels "good, kindhearted").

This is one of the few instances in which an original long e (ee) has become shortened to i. The same change occurs in breeches, and in the American pronunciation of been, with no change in spelling. [Century Dictionary]

The word's considerable sense development moved from "happy" to "blessed" to "pious," to "innocent" (c. 1200), to "harmless," to "pitiable" (late 13c.), "weak" (c. 1300), to "feeble in mind, lacking in reason, foolish" (1570s). Further tendency toward "stunned, dazed as by a blow" (1886) in knocked silly, etc. Silly season in journalism slang is from 1861 (August and September, when newspapers compensate for a lack of hard news by filling up with trivial stories). Silly Putty trademark claims use from July 1949.

It is a widespread phenomenon that the words for 'innocent', apart from their legal use, develop, through 'harmless, guileless', a disparaging sense 'credulous, naive, simple, foolish.' [Buck]
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