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.
"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 Frisianthekka, 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 þeccan(see thatch (v.), which is a doublet).Related: Decked; decking.
"to knock down," by 1955, probably from deck (n.) on the notion of laying someone out on a ship's deck. Compare floor (v.) "to knock down." Related: Decked; decking.