Inapt and inept mean "not apt," "not fitting," "without aptitude or capacity": "This person is inapt in computing the sales tax on purchases." "Tom is a good mechanic, but he is inept in keeping customers satisfied." Inapt and inept share such approximate synonyms as unsuited, inappropriate, awkward, and clumsy. The exact opposite (antonym) of inapt is capable; that of inept is suited. Inapt is more commonly used than inept in the meanings indicated above. Unapt also has meanings of "inappropriate" and "unsuitable" but is more often used to mean "not likely" or "not disposed": "The supervisor is unapt to let you leave early."
Dictionary of problem words and expressions. Harry Shaw. 1975.