deficient

Etymology

From Latin deficiens, present participle of deficere (“to lack, fail, be wanting”); see defect.

adj

1. Lacking something essential; often construed with in.
   They were deficient in social skills.
2. Insufficient or inadequate in amount.
3. (mathematics) Of a number n, Having the sum of divisors σ(n)<2n, or, equivalently, the sum of proper divisors (or aliquot sum) s(n)<n.

noun

1. A person who is deficient.
   This was justified by the rationale that mental deficients, by definition, are not able to succeed in society. 2004, Steven Noll, James Trent, Mental Retardation in America: A Historical Reader, page 316