from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
IRREVOCABLE. That which cannot be revoked.
2. A will may at all times be revoked by the same person who made it,
he having a disposing mind; but the moment the testator is rendered
incapable to make a will he can no longer revoke a former will, because he
wants a disposing mind. Letters of attorney are generally revocable; but
when made for a valuable consideration they become irrevocable. 7 Ves. jr.
28; 1 Caines' Cas. in Er. 16; Bac. Ab. Authority, E. Vide Authority;
License; Revocation.