the.com/brill

British slang for brilliant, worn down by enthusiasm like a cheap heel.

means An informal British shortening of 'brilliant,' used to mean excellent or great.

from Simply a clipping of 'brilliant' — English loves to hack the ends off words that get used too often ('totes', 'obvs', 'def'), and 'brill' caught on in British slang especially from the mid-to-late 20th century, often associated with the chirpy enthusiasm of kids' telly and playground talk. No hidden backstory here: it's just 'brilliant' with the back half worn off from overuse.

origin
Clipped from brilliant, 1970s-80s British playgrounds
also means
A flatfish related to turbot, confusingly
register
Sounds sincere or sarcastic depending entirely on tone
export status
Rarely survives crossing the Atlantic intact
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