borrowed from Latin dēficient-, dēficiens, present participle of dēficere "to leave without enough, let down, be lacking, run short, fail," from dē- de- + facere "to make, bring about, perform, do" — more at fact : lacking something necessary for completeness or health Her diet is deficient in proteins.

lacking or not good enough: A diet that is deficient in protein is harmful to children. (Definition of deficient from the Cambridge Academic Content Dictionary © Cambridge University Press)