Acting instinctively means doing something without thinking because it comes naturally to you—like catching a ball thrown your way or dodging out of danger.
The adverb instinctively comes from the noun instinct, which refers to "an innate, natural response." When you react instinctively, you’re relying on automatic impulses or "gut feelings," often without realizing it. For instance, if you instinctively reach out to catch a falling object or flinch when startled, your brain is bypassing conscious thought to help keep you safe.