The antonyms for the word "most smashing" would be words that convey the opposite meaning. These words could be dull, unremarkable, lackluster, or mediocre. They could also be words that suggest destruction or damage, such as ruinous, devastating, or shattering. Other antonyms might include words that suggest restraint or moderation, such as tame, subtle, or understated. Ultimately, the antonyms for "most smashing" would be words that evoke a sense of modesty, ordinariness, or even negativity - highlighting the contrast between the two opposing concepts.