"covering over part of a ship," mid-15c., perhaps a shortening of Middle Low German verdeck (or a related North Sea Germanic word), a nautical word, from ver- "fore" + decken "to cover, put under roof," from Proto-Germanic *thackjam (related to thatch, q.v.).
Sense extended early in English from "covering" to "platform of a ship." "Pack of cards" is 1590s, perhaps because they were stacked like decks of a ship. Deck chair (1884) so called because they were used on ocean liners. Tape deck (1949) is in reference to the flat surface of old reel-to-reel tape recorders.
"adorn" (as in deck the halls), early 15c., from Middle Dutch dekken "to cover," from the same Germanic root as deck (n.). Meaning "to cover" is from 1510s in English. Replaced Old English þeccan. Related: Decked; decking.
"knock down," c.1953, probably from deck (n.) on the notion of laying someone out on the deck. Related: Decked; decking.
To knock someone down, esp with the fist; floor: Remember that guy I decked in the restaurant? (1940s+)