And apparently (according to the GPC) “broga” does come from the English. In fact, “ffroga” is recorded in Welsh before “broga”.
The “ff” to “b” change is a bit of an odd one (“f” to “b” or “m” is perfectly understandable and common)* but “ff” seems to be a bit of a wild card sometimes! (Ffebruar to chwefror etc!)
- that is, the sound represented in Welsh by “f”, and similarly with “ff”.s