1. A basis, ground, or premise that lacks legal validity, proper authorization, or moral justification
The court ruled that the contract had an illegitimate foundation and was therefore void.
O tribunal decidiu que o contrato tinha uma fundação ilegítima e, portanto, era nulo.
2. An argument, claim, or theory that rests on false, unproven, or unjustifiable premises
The entire thesis was built on an illegitimate foundation of unsupported assumptions.
Toda a tese foi construída sobre uma fundação ilegítima de pressupostos não comprovados.