Tester has to find the bug without proper planning just based on the assumptions. It is the form of black box testing because tester has not idea of the internal code and structure. Adhoc test done by the skillful tester and considered as the best techniques to uncovering the certain types of defects.
It is also called the monkey testing. In this type of testing we don’t have any specification but we aware of its functionality .In this we will give value randomly & perform task without thinking of its ill effect.