Step 1: Understanding the Concept:
The question asks to identify the case that provided the broad, non-technical definition of "misconduct" for professionals. The definition cited emphasizes the judgment of peers ("professional brethren of good repute") in determining what constitutes disgraceful or dishonorable conduct.
Step 2: Detailed Explanation:
The definition of 'misconduct' quoted in the question is a classic legal formulation. It was authoritatively laid down by the Supreme Court in the case of Noratanmal Chouraria v. M.R. Murli, (2004) 5 SCC 689. The court, in this case, was dealing with the issue of professional misconduct by an advocate. It held that the term 'misconduct' is not a precise term with a specific legal meaning. It is an elastic concept that has to be judged based on the facts and circumstances of each case. The court approved the test laid down in earlier cases that misconduct is conduct that would be reasonably regarded as disgraceful or dishonorable by one's professional peers.
- Bar Council of Maharashtra v. M.V. Dabholkar is a very important case on professional ethics, particularly concerning advertising by advocates, but the specific quote is from \textit{Noratanmal Chouraria}.
- N.G. Dastane v. Shrikant S. Shinde also deals with professional misconduct, but it's more known for clarifying that even a single act can sometimes amount to misconduct.
- B. M. Verma v. Uttrakhand Regulatory Commission is not a primary landmark case on this point.
Step 3: Final Answer:
The definition was conclusively established in the case of Noratanman Courasia v. M. R. Murali.