Only, conj.
Save or except (that); -- an adversative used elliptically with or without that, and properly introducing a single fact or consideration. He might have seemed some secretary or clerk . . . only that his low, flat, unadorned cap . . . indicated that he belonged to the city. Sir W. Scott.
2 other Webster entries under this spelling
Only, a.
1. One alone; single; as, the only man present; his only occupation.
2. Alone in its class; by itself; not associated with others of the same class or kind; as, an only child.
3. Hence, figuratively: Alone, by reason of superiority; preëminent; chief. "Motley's the only wear." Shak.
Only, adv.
1. In one manner or degree; for one purpose alone; simply; merely; barely. And to be loved himself, needs only to be known. Dryden.
2. So and no otherwise; no other than; exclusively; solely; wholly. "She being only wicked." Beau. & Fl. Every imagination . . . of his heart was only evil. Gen. vi. 5.
3. Singly; without more; as, only-begotten.
4. Above all others; particularly. [Obs.] His most only elected mistress. Marston.
Shades of meaning
Fernald has no entry of its own for only; he lists it under but.
He lists it without comment; the entries above explain its neighbors.