Astucious is an adjective that describes someone who is cunning or sly. Antonyms for this word would be honest, straightforward, naive, or dull. Someone who is honest and straightforward would be the opposite of astucious, as they are not trying to deceive or trick others. Naive might also be an antonym, as it implies someone who is innocent and lacking in experience or knowledge of the world. Dull, which means lacking in sharpness, brightness, or interest, would be an antonym when used to describe a person's personality or intellect, as it implies someone who is not quick-witted or clever.