Ungullible is a word commonly used to describe someone who is not easily fooled or deceived by others. There are various synonyms for this term, including shrewd, astute, judicious, discerning, perceptive, cautious, insightful, and vigilant. Each of these synonyms highlights different aspects of being un-gullible. Shrewd and astute suggest someone who is sharp and quick-witted in their observations. Judicious and discerning imply a thoughtful, careful consideration of possible outcomes before making a decision. Meanwhile, perceptive and insightful convey an ability to see through things and understand people's intentions. Lastly, cautious and vigilant emphasize the importance of being wary and alert to avoid being tricked.