The Collaborative International Dictionary of English v.0.48:

Inadvisable \In`ad*vis"a*ble\, a.
   Not advisable. -- In`ad*vis"a*ble*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inadvisable
    adj 1: not prudent or wise; not recommended; "running on the ice
           is inadvisable" [syn: inadvisable, unadvisable] [ant:
           advisable]
    2: not advisable; "an unnecessary and inadvisable action"

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "inadvisable":
   bad, careless, foolhardy, foolish, futile, harebrained,
   ill-advised, ill-considered, ill-contrived, ill-devised,
   ill-gauged, ill-judged, ill-suited, ill-timed, impolitic, improper,
   imprudent, inappropriate, inapt, incautious, incongruous,
   inconsiderate, indiscreet, inept, inexpedient, infelicitous,
   injudicious, inopportune, insensate, irrational, mal a propos,
   malapropos, mindless, misadvised, misguided, myopic, out of place,
   pointless, rash, reasonless, reckless, senseless, shortsighted,
   thoughtless, unadvised, unbefitting, unconsidered, undesirable,
   undiscerning, unfit, unfitting, unforeseeing, unfortunate, unhappy,
   unmeet, unprofitable, unreasonable, unreflecting, unreflective,
   unseasonable, unseeing, unseemly, unsensible, unsound, unsuitable,
   unthinking, unthoughtful, untimely, unwise, witless, wrong