late 14c., "having no companion, solitary, apart from any other," shortening of alone (q.v.) by weakening of stress or else by misdivision of what is properly all one. Used attributively, while the full form is used in the predicate. Compare live (adj.), from alive; colloquial 'long for along. The Lone Star in reference to Texas is first recorded 1843, from its flag when it was a nation. Lone wolf in the figurative sense is 1901, American English.
"one who avoids company," 1946; see lone. Apparently first in U.S. baseball slang:
Ted [Williams] is likable enough in spite of his obsession with his specialty. He is something of a "loner," and he refuses to pal around with his teammates in off hours, but in the clubhouse he does his share of the talking. [Life magazine, Sept. 23, 1946]
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]
In other European languages, identity between the indefinite article and the word for "one" remains explicit (French un, German ein, etc.). Old English got by without indefinite articles: He was a good man in Old English was he wæs god man.
In texts of Shakespeare, etc., an as a word introducing a clause stating a condition or comparison conjunction is a reduced form of and in this now-archaic sense "if" (a usage first attested late 12c.), especially before it.