The phrase "most rascally" typically refers to someone who is mischievous, impish, or even villainous. There are a variety of synonyms that could be used in place of "most rascally," depending on the context. For example, if you're talking about a clever thief or con artist, you might use words like "sly," "devious," or "cunning." If you're referring to someone who is more reckless or impulsive in their misbehavior, you could describe them as "wild," "rowdy," or "untamed." Other possible synonyms for "most rascally" might include "rogueish," "scampish," or "wayward," each of which adds a slightly different nuance to the original phrase.