"adorn, array or clothe with something ornamental" (as in deck the halls), early 15c., from Middle Dutch decken "to cover, put under roof," a nautical word, from Proto-Germanic *thakjan (source also of Old Frisian thekka, Old High German decchan, German decken), from PIE root *(s)teg- "to cover." Meaning "to cover, overspread" is from 1510s in English. Replaced Middle English thecchen, from Old English eccan(see thatch (v.), which is a doublet).Related: Decked; decking.

Proto-Indo-European root meaning "to cover," especially with a roof. It forms all or part of: deck (n.) "covering over part of a ship;" deck (v.) "adorn;" deckle; detect; integument; protect; protection; stegosaurus; tegular; tegument; thatch; thug; tile; Tuileries.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sthag- "cover, conceal, hide;" Greek stegein "to cover," stegos "a roof;" Latin tegere "to cover," tegula "tile;" Lithuanian stėgti "to roof;" Old Norse þekja, Old English þeccan "thatch;" Dutch dekken, German decken "to cover, put under roof;" Irish tuigiur "cover," tech "house;" Welsh toi "thatch, roof," ty "house."

mid-15c., dekke, "covering extending from side to side over part of a ship," from a nautical use of Middle Dutch dec, decke "roof, covering," from Proto-Germanic *thakam (source also of thatch (n.)), from PIE root *(s)teg- "to cover."

Sense extended early in English from "covering" to "platform of a ship." Meaning "pack of cards necessary to play a game" is from 1590s, perhaps because they were stacked like decks of a ship. Tape-deck (1949) is in reference to the flat surface of old reel-to-reel tape recorders. 

Deck-chair (1844) so called because they were used on ocean liners. On deck (by 1740) was in nautical use especially "ready for action or duty;" extended sense in baseball, of a batter waiting a turn at the plate, is by 1867. To clear the deck (1852) is to prepare a ship for action; it is perhaps a translation of French débarasser le pont.

1787, "pointed or blunt-pointed instrument used in prodding;" 1802, "act of prodding;" from prod (v.). A provincial word; it also meant "long wooden pin used to secure thatch on a roof."

"outer upper covering of a house or other building," Middle English rof, from Old English hrof "roof," also "ceiling," hence figuratively "highest point, top, summit" also "heaven, the sky;" from Proto-Germanic *khrofam (source also of Old Frisian rhoof "roof," Middle Dutch roof, rouf "cover, roof," Dutch roef "deckhouse, cabin, coffin-lid," Middle High German rof "penthouse," Old Norse hrof "boat shed").

No apparent connections outside Germanic. "English alone has retained the word in a general sense, for which the other languages use forms corresponding to OE. þæc thatch" [OED]. Meaning "top of a carriage, etc." is by 1706. The meaning "upper part of the mouth, the hard palate" was in late Old English (hrof ðæs muðes). To raise the roof "create an uproar" is attested from 1860, originally in U.S. Southern dialect.