only (adj.)

"single as regards number, class, or kind," Middle English onli, from Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Similar formation in Old Frisian einlik, Dutch eenlijk, Old High German einlih, Danish einlig. It preserves the old pronunciation of one. Related: Onliness.

Its use as an adverb ("alone, no other or others than; in but one manner; for but one purpose") and conjunction ("but, except") 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 word was ancenned. Only child is attested by 1700.

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Definitions of only
only (adv.)
and nothing more;
he was only a child
Synonyms: merely / simply / just / but
only (adv.)
without any others being included or involved;
a privilege granted only to him
Synonyms: entirely / exclusively / solely / alone
only (adv.)
with nevertheless the final result;
He arrived only to find his wife dead
We won only to lose again in the next round
only (adv.)
in the final outcome;
These news will only make you more upset
only (adv.)
except that; "It was the same story; only this time she came out better";
only (adv.)
never except when;
call me only if your cold gets worse
Synonyms: only if / only when
only (adv.)
as recently as;
I spoke to him only an hour ago
only (adj.)
being the only one; single and isolated from others;
an only child
Synonyms: lone / lonesome / sole / solitary
only (adj.)
exclusive of anyone or anything else;
I'll have this car and this car only
Synonyms: alone