Leniently means in a way that is not strict: how someone punishes, or how high a standard they insist on. The school treated the first mistake leniently. The judge ruled leniently. It is the same idea as lenient, but it tells you about the action, not the person. Though actually, in very casual speech some people drop the -ly (“they treated him lenient”), but careful English keeps it: treated leniently. The word still lives in schools, courts, and work — anywhere a rule could have been harder and was not. It can sound fair, or it can sound like someone got off too lightly; the sentence around it decides.