The Collaborative International Dictionary of English v.0.48:

Thankful \Thank"ful\, a. [AS. [thorn]ancfull.]
   1. Obtaining or deserving thanks; thankworthy. [R.]
      [1913 Webster]

            Ladies, look here; this is the thankful glass
            That mends the looker's eyes; this is the well
            That washes what it shows.            --Herbert.
      [1913 Webster]

   2. Impressed with a sense of kindness received, and ready to
      acknowledge it; grateful.
      [1913 Webster]

            Be thankful unto him, and bless his name. --Ps. c.
                                                  4.
      [1913 Webster] -- Thank"ful*ly, adv. --
      Thank"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thankful
    adj 1: feeling or showing gratitude; "a grateful heart";
           "grateful for the tree's shade"; "a thankful smile" [syn:
           grateful, thankful] [ant: thankless, ungrateful,
           unthankful]