Resembling, containing, or related to birds; or in golf, scoring one stroke under par on a hole.
From 'bird' plus the suffix '-y' meaning 'resembling or containing.' The golf term 'birdie' (related spelling) originated in early 20th-century American golf slang, where 'bird' was slang for something excellent, and scoring well became a 'birdie.'
Golf slang created 'birdie' because 1920s golfers called good things 'birds' (like how we say 'cool')—so a one-under score was 'a bird of a score,' which got shortened to 'birdie.' It's the same reason an even-better score is an 'eagle'!
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