Instinctively means without stopping to think. The body decided before the mind was consulted.
It attaches to reactions, so instinctively ducking, instinctively reaching out, instinctively knowing something is wrong. That last use is looser, since instinctive knowledge is really fast pattern recognition rather than instinct proper. Biology reserves the noun instinct for inherited behaviour, like a bird building a nest. In everyday English it covers anything that feels automatic, and gut feeling is the informal version.
1adverbinstinctivelyWithout conscious thought, as an automatic reaction.
“He instinctively raised an arm to protect his face.”
From instinctive + -ly.
Etymology from Wiktionary, CC BY-SA