Demeanor, n. [Written also demeanour.]
Etymology: [For demeanure, fr. demean. See Demean, v. t.]
1. Management; treatment; conduct. [Obs.] God commits the managing so great a trust . . . wholly to the demeanor of every grown man. Milton.
2. Behavior; deportment; carriage; bearing; mien. His demeanor was singularly pleasing. Macaulay. The men, as usual, liked her artless kindness and simple refined demeanor. Thackeray.
Shades of meaning
Fernald has no entry of its own for demeanor; he lists it under air, behavior.
Where he writes about it:
Mien may be used to express some prevailing feeling; as, "an indignant mien." Demeanor goes beyond appearance, including conduct, behavior; as, a modest demeanor. — from air
Demeanor is the bodily expression, not only of feelings, but of moral states; as, a devout demeanor. — from behavior