None but the very ignorant would say onliest, but one often sees the expressions more and most unique.
Imogene, my onliest, sweet bird of paradise, it is your mate who calls.
My ma an her chillen wuz the onliest slaves my marster and mistis had.
I come back to see my darter, the onliest chile I got in the worl'.
Got my ol' blue-back Webster, onliest book I evah had, 'scusin' mah Bible.
Burying in dirt is the onliest thing that'll take off the smell.
All the rest of them is dead and he is the onliest one that is living.
Why, Ellen girl, don't you know that ever since the first day I saw you you've been the onliest girl for me!
De boys would wuk fer dey money but I wuz the onliest gal an' Mammy wouldn't let me go off de plantation to make none.
I tell you bout the onliest man I knowed come from Virginia.
Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Use as an adverb and conjunction developed in Middle English. Distinction of only and alone (now usually in reference to emotional states) is unusual; in many languages the same word serves for both. German also has a distinction in allein/einzig. Phrase only-begotten (mid-15c.) is biblical, translating Latin unigenitus, Greek monogenes. The Old English form was ancenned.