Etymology
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birthday (n.)

late 14c., from Old English byrddæg, "anniversary or celebration of one's birth" (at first usually a king or saint); see birth (n.) + day. Meaning "day on which one is born" is from 1570s. Birthnight is attested from 1620s.

updated on June 02, 2017

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Definitions of birthday from WordNet

birthday (n.)
an anniversary of the day on which a person was born (or the celebration of it);
birthday (n.)
the date on which a person was born;
Synonyms: natal day
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.