'(Br.)' originates from English, specifically the word 'British', where 'British' derives ultimately from Latin 'Britannia', the name for the island (the land of the Britons).

'British' changed from Old English 'Bryttisc' (from 'Bryttas', the Britons) via Middle English and later became the modern English adjective 'British'; the abbreviated label 'Br.' / '(Br.)' developed as a dictionary usage label to indicate British usage.

Initially it meant 'of or relating to the Britons or Britain'; over time it retained that meaning and additionally became a conventional label indicating British English usage in dictionaries.