There are a plethora of synonyms for the phrase "silly girl." Depending on the context, the phrase can hold different connotations, and there are multiple words that can be used to capture its essence. Some alternatives to "silly girl" include "airy-headed," "scatterbrained," "thoughtless," and "frivolous." Other possibilities might include "naive," "childish," "foolish," or even "giddy." Although they might not always have a negative connotation, these words carry the same implication of someone who might not take things seriously or who is easily distracted by trivialities. In any case, the kind of word used to describe a "silly girl" will depend on the context and tone of conversation.