The Collaborative International Dictionary of English v.0.48:

Underlay \Un`der*lay"\, v. t. [AS. underlecgan. See Under, and
   Lay, v. t.]
   1. To lay beneath; to put under.
      [1913 Webster]

   2. To raise or support by something laid under; as, to
      underlay a cut, plate, or the like, for printing. See
      Underlay, n., 2.
      [1913 Webster]

   3. To put a tap on (a shoe). [Prov. Eng.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underlay \Un`der*lay"\, v. i. (Mining)
   To incline from the vertical; to hade; -- said of a vein,
   fault, or lode.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underlay \Un"der*lay`\, n.
   1. (Mining) The inclination of a vein, fault, or lode from
      the vertical; a hade; -- called also underlie.
      [1913 Webster]

   2. (Print.) A thickness of paper, pasteboard, or the like,
      placed under a cut, or stereotype plate, or under type, in
      the form, to bring it, or any part of it, to the proper
      height; also, something placed back of a part of the
      tympan, so as to secure the right impression.
      [1913 Webster]

WordNet (r) 3.0 (2006):

underlay
    n 1: a pad placed under a carpet [syn: carpet pad, rug pad,
         underlay, underlayment]
    v 1: raise or support (the level of printing) by inserting a
         piece of paper or cardboard under the type; "underlay the
         plate"
    2: put (something) under or beneath; "They underlaid the
       shingles with roofing paper"
    3: provide with a base, support, lining, or backing; "underlay
       the boards with joists"