The Collaborative International Dictionary of English v.0.48:

Junk \Junk\, n. [Pg. junco junk, rush, L. juncus a bulrush, of
   which ropes were made in early ages. Cf. Junket.]
   [1913 Webster]
   1. Pieces of old cable or old cordage, used for making
      gaskets, mats, swabs, etc., and when picked to pieces,
      forming oakum for filling the seams of ships.
      [1913 Webster]

   2. Old iron, or other metal, glass, paper, etc., bought and
      sold by junk dealers.
      [1913 Webster]

   3. Hence: Something worthless, or only worth its value as
      recyclable scrap.
      [PJC]

   4. (Naut.) Hard salted beef supplied to ships.
      [1913 Webster]

   Junk bottle, a stout bottle made of thick dark-colored
      glass.

   Junk dealer, a dealer in old cordage, old metal, glass,
      etc.

   Junk hook (Whaling), a hook for hauling heavy pieces of
      blubber on deck.

   Junk ring.
      (a) A packing of soft material round the piston of a steam
          engine.
      (b) A metallic ring for retaining a piston packing in
          place;
      (c) A follower.

   Junk shop, a shop where old cordage, and ship's tackle, old
      iron, old bottles, old paper, etc., are kept for sale.

   Junk vat (Leather Manuf.), a large vat into which spent tan
      liquor or ooze is pumped.

   Junk wad (Mil.), a wad used in proving cannon; also used in
      firing hot shot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

junk shop
    n 1: a shop that sells cheap secondhand goods