"brathly" is a regional or dialect word meaning hastily; violently; fiercely; with fury. It is most likely picked up from family, a particular part of the country, or a piece of regional media. There is nothing to worry about: dialect vocabulary is a sign of place and heritage, not of any risky behaviour. If anything, it is a nice opportunity to talk about where the word comes from and which side of the family or which place uses it.