WordNet (r) 3.0 (2006):

dig in
    v 1: occupy a trench or secured area; "The troops dug in for the
         night" [syn: entrench, dig in]
    2: eat heartily; "The food was placed on the table and the
       children pitched in" [syn: pitch in, dig in]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "dig in":
   arm, armor, armor-plate, bank, barricade, battle, blockade,
   bulwark, castellate, crenellate, embattle, entrench, fence,
   fortify, garrison, hang in, hang in there, hang tough, hold fast,
   hold out, man, man the garrison, mine, palisade, remain firm,
   stand fast, stand firm, stand no nonsense, stand the gaff, stick,
   stick fast, take what comes, wall