WordNet (r) 3.0 (2006):

on the side
    adv 1: without official authorization; "he had made some money
           on the side" [syn: unofficially, on the side]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "on the side":
   above, additionally, again, all included, also, altogether,
   among other things, and all, and also, and so, as well, aside,
   au reste, away, beside, besides, beyond, else, en plus, extra,
   farther, for lagniappe, further, furthermore, in addition,
   in juxtaposition, inter alia, into the bargain, item, likewise,
   more, moreover, nearby, on one side, on top of, over, plus,
   sidelong, similarly, then, therewith, to boot, to one side,
   to the side, too, yet