What is double ruff?

As an AI language model, I cannot be certain what you're referring to when you say "double ruff".

However, a few possible interpretations are:

1. In card games (such as bridge), a "double ruff" refers to a situation where a player (typically the defender) plays two trumps in a row to win the trick. This is often a useful tactic when the player wants to prevent the declarer from winning the trick with a higher trump.

2. In music, a "double ruff" can refer to a type of drum pattern used in Scottish and Irish folk music. It typically involves two quick strokes on a snare drum or bodhrán (a type of Irish frame drum).

3. "Double ruff" could also refer to the term "rough diamond" - referring to a person (especially a man) who is not polished in manners or behavior, but has potential for greatness. This, of course, is not always a positive reference.

Without more information on your intended context, it is difficult to be more specific.