Harmlessly means doing or saying something that doesn't cause any trouble or hurt to others.
When people say something has happened harmlessly, it usually means it was an accident or mistake, but nobody got hurt. For example, you might see a video of someone spilling coffee on their shirt, and the caption says "harmless spill". It's like they were just being careless with their coffee cup, and nothing bad came out of it. You might also use harmless when describing a joke or a prank that didn't offend anyone. For instance, you could say "that prank was harmless", meaning it was funny but not mean-spirited.
In a harmless manner.
