The phrase "casual reference" describes something that is mentioned briefly and without much emphasis. There are several synonyms that can be used in place of this phrase. One option is "passing mention," which suggests that something was brought up in a passing or incidental way. Another synonym for "casual reference" is "offhand remark," which implies that something was mentioned without much thought or intention. "Incidental comment" is another possible synonym, suggesting that something was mentioned incidentally or in passing. Other options include "brief allusion" or "casual mention." All of these terms can be used to describe something that is mentioned briefly and without much emphasis or weight.