The phrase "on last leg" is typically used to describe a situation where something is coming to an end or is in a state of decline. Antonyms for this phrase could include words and phrases that indicate a sense of strength, resilience, and vitality. Words like "strong," "enduring," "vibrant," "robust," and "blossoming" are all antonyms that could be used when describing something that is not on its last leg. These words connote a sense of energy and vitality, indicating that something is not coming to an end, but rather is just beginning to flourish. When looking for an antonym to "on last leg," try to focus on words that convey the opposite meaning: strength, growth, and vitality.