Insufficient or inadequate in amount.
Lacking something essential; often construed with in.
Of a number n, Having the sum of divisors σ(n)<2n, or, equivalently, the sum of proper divisors (or aliquot sum) s(n).
A person who is deficient.
Unclear or unreliable.
Experiencing or showing doubt, sceptical.
Subject to, or causing doubt.
Improbable or unlikely.
Suspicious, or of dubious character.
Undecided or of uncertain outcome.
A doubtful person or thing.