Antonyms for the phrase "think likely" could include "doubtful," "unlikely," or "inconceivable." When someone expresses that they think something is likely, they are typically indicating that they believe it is probable or quite possible. By contrast, the antonyms of this phrase suggest a lack of confidence or belief in the likelihood of a particular event or outcome. These words may be used to convey skepticism, disbelief or uncertainty in relation to a statement or claim made by someone else, or to express one's own doubts or reservations about a particular situation or event.