"drearily solitary; secluded from society; dejected from want of company," 1640s, from lone (adj.) + -some (1). Related: Lonesomeness. An older adjective was loneful (1560s).
Loneliness expresses the uncomfortable feelings, the longing for society, of one who is alone. Lonesomeness may be a lighter kind of loneliness, especially a feeling less spiritual than physical, growing out of the animal instinct for society and the desire of protection, the consciousness of being alone .... Lonesomeness, more often than loneliness, may express the impression made upon the observer. [Century Dictionary]
"state of being alone, remoteness from society," mid-14c., from Old French solitude "loneliness" (14c.) and directly from Latin solitudinem (nominative solitudo) "loneliness, a being alone; lonely place, desert, wilderness," from solus "alone" (see sole (adj.)). "Not in common use in English until the 17th c." [OED]
A man can be himself only so long as he is alone; ... if he does not love solitude, he will not love freedom; for it is only when he is alone that he is really free. [Schopenhauer, "The World as Will and Idea," 1818]
Solitudinarian "recluse, unsocial person" is recorded from 1690s.
mid-14c., solitarie, "alone, by oneself or itself, living alone," from Anglo-French solitarye and Old French solitaire, from Latin solitarius "alone, lonely, isolated," from solitas "loneliness, solitude," from solus "alone" (see sole (adj.)).
Of places, "remote from society, unfrequented," late 14c. The meaning "single, sole, only" is by 1742. Related: Solitarily; solitariness. Solitary confinement "separate confinement of a prisoner with limited access of other persons" is by 1690s; short form solitary for this is by 1854.