In law, if something you say or do is actionable, it is bad or damaging enough that the other person could take you to court over it. A lie that ruins someone’s name, or a careless act that hurts them, might be actionable: it can support a claim in court. Lawyers and news reports use this; it is formal. The able here means “can be the basis of an action,” and action in court is a case, not “doing something.” Though actually, outside courtrooms the word has drifted. In offices, actionable advice or actionable data now usually means “we can do something with this; it is practical, not just talk.” That newer sense is everywhere in business English, and it is the one students will hear more. Same spelling, almost the opposite angle: one is “you could be sued,” the other is “you can act on it.” This entry is the court meaning.