Producing flowers; flourishing or developing successfully; used as a mild intensifier (British).
From Old Norse 'blóm' meaning flower, related to 'blow' (to blossom). The intensifier use ('blooming idiot') developed in 19th-century British English as a euphemism for 'bloody', considered too strong for polite conversation.
The British use of 'blooming' as a mild swear word shows how floral metaphors can become surprisingly expressive. It's part of a whole garden of British euphemisms - 'blinking', 'blasted', 'ruddy' - that let people curse while sounding almost poetic.
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