WordNet sense 1 (someone who is not competent to take effective action):

incompetent person

WordNet sense 2 (showing lack of skill or aptitude):

bungling, clumsy, fumbling

WordNet sense 3 (not having or showing or requiring special skill or proficiency):

unskilled

WordNet sense 4 (having undesirable or negative qualities):

bad

WordNet sense 5 (lacking specific legal qualifications):

unqualified

WordNet sense 6 (lacking the ability or skill to perform effectively; inadequate):

inefficient

WordNet sense 7 (lacking the necessary skill or knowledge etc.):

incapable

WordNet sense 8 (generally incompetent and ineffectual):

inept

WordNet sense 9 ((sometimes followed by `to') not meeting the requirements especially of a task):

inadequate