Antonyms for the phrase "knocks props from under" include words that convey the opposite meaning, such as "support", "uphold", "bolster", "strengthen", and "sustain". These words imply the act of providing assistance, encouragement, or reinforcement, rather than undermining or weakening someone or something. Using antonyms such as "build up", "encourage", "cheer", "fortify", and "inspire" can help convey positivity and encouragement. Conversely, using synonyms such as "undermine", "weaken", "dishearten", "discourage" and "sap" can contribute to a negative connotation and imply a damaging or harmful effect. Therefore, it is essential to choose the appropriate word when expressing a specific sentiment or intention.