Inconsistent, a.
Etymology: [Pref. in- not + consistent: cf. F. inconsistant.]
1. Not consistent; showing inconsistency; irreconcilable; discordant; at variance, esp. as regards character, sentiment, or action; incompatible; incongruous; contradictory.
Compositions of this nature . . . show that wisdom and virtue are far from being inconsistent with politeness and good humor. Addison.
2. Not exhibiting uniformity of sentiment, steadiness to principle, etc.; unequal; fickle; changeable. Ah, how unjust to nature, and himself, Is thoughtless, thankless, inconsistent man. Young.
Syn. -- Incompatible; incongruous; irreconcilable; discordant; repugnant; contradictory. -- Inconsistent, Incongruous, Incompatible. Things are incongruous when they are not suited to each other, so that their union is unbecoming; inconsistent when they are opposed to each other, so as render it improper or wrong; incompatible when they can not coexist, and it is therefore impossible to unite them. Habitual levity of mind is incongruous with the profession of a clergyman; it is inconsistent with his ordination vows; it is incompatible with his permanent usefulness. Incongruity attaches to the modes and qualities of things; incompatibility attaches to their essential attributes; inconsistency attaches to the actions, sentiments, etc., of men.
Shades of meaning
Fernald has no entry of its own for inconsistent; he lists it under incongruous.
Where he writes about it:
Incompatible primarily signifies unable to sympathize or feel alike; inconsistent means unable to stand together. — from incongruous
Inconsistent applies to things that can not be made to agree in thought with each other, or with some standard of truth or right; slavery and freedom are inconsistent with each other in theory, and incompatible in fact. — from incongruous