WordNet sense 1 (a cruelly rapacious person):

beast, brute, savage, wolf

WordNet sense 2 (an exploratory oil well drilled in land not known to be an oil field):

wildcat well

WordNet sense 3 (not sound financially):

unsound

WordNet sense 4 (without official authorization):

unauthorized, unauthorised

WordNet sense 5 (not having official authority or sanction):

unofficial

WordNet sense 6 (serving in or intended for exploration or discovery):

exploratory, explorative