The Collaborative International Dictionary of English v.0.48:

sandbag \sand"bag`\, n.
   A bag filled with sand; small sandbags may be used as a
   weapon, or larger ones to build walls or as ballast; as, they
   kept the flooding river from the area by buiding a temmporary
   dike out of sandbags.
   [WordNet 1.6]

The Collaborative International Dictionary of English v.0.48:

sandbag \sand"bag`\, v.
   To treat harshly or unfairly. [WordNet sense 1]
   [WordNet 1.6]

   2. To hit something or somebody with or as if with a sandbag.
      [WordNet sense 2]
      [WordNet 1.6]

   3. To protect or strengthen with sandbags; stop up; as, the
      residents sandbagged the beach front. [WordNet sense 3]
      [WordNet 1.6]

   4. To thwart (another person's plans) by surreptitious
      maneuvers; as, he sandbagged my proposal by talking in
      private with other members of the committee. [informal]
      [PJC]

   5. To intimidate or coerce, especially by crude methods.
      [Informal]
      [PJC]

   6. To deceive and take advantage of (a person) by
      misrepresenting one's true intentions. [Informal]
      [PJC]

   7. Hence: (Poker) To encourage opponents into betting more by
      first refraining from betting while having a strong hand,
      and only later raising the stakes. In informal games,
      certain types of sandbagging are forbidden.
      [PJC]

WordNet (r) 3.0 (2006):

sandbag
    n 1: a bag filled with sand; used as a weapon or to build walls
         or as ballast
    v 1: treat harshly or unfairly
    2: compel by coercion, threats, or crude means; "They sandbagged
       him to make dinner for everyone" [syn: dragoon, sandbag,
       railroad]
    3: hit something or somebody as if with a sandbag [syn:
       sandbag, stun]
    4: downplay one's ability (towards others) in a game in order to
       deceive, as in gambling
    5: protect or strengthen with sandbags; stop up; "The residents
       sandbagged the beach front"