The phrase "get off one's chest" refers to expressing one's thoughts, emotions, or feelings that have been bothering them for some time. Some of the antonyms for this phrase include repress, suppress, withhold, and conceal. Repressing or suppressing one's thoughts or feelings can lead to anxiety, stress, and even depression. Holding back one's emotions may also lead to resentment and bitterness, which can affect personal relationships. Thus, it is always advisable to express oneself honestly and openly to maintain mental and emotional stability. So, instead of concealing your thoughts or feelings, try to confide in a trusted friend or seek the help of a mental health professional to get the support you need.