Dropping the '-ly' in adverbs used in a negative response to a question

Two British gangsters riding in London traffic. One says: 'Have you seen him?' Other says: 'Not recent. One says: 'do you still go there?' Other says: 'Not regular.' Is this a colloquialism, idiom or just plain bad grammar.

If people often say it and people often do it, then it's not bad grammar. It might be a usage from a specific regional variety of English, in which case it will be regarded as non-standard, which in reality just means 'different'. This, does not constitute 'bad grammar'.

[On the other hand, if was being made up by a writer doing an inaccurate impression of a London gangster, then of course, in this case, it would just be bad grammar!]

I live in London most of the year and I've never heard this usage. However, this does not mean that it's not authentic. Comments here suggest that it is indeed a feature of certain varieties, which would indicate that it's a grammatical form of a specific type of English.

Hope this helps!