"Passing strange" is a unique phrase that is often used to describe things that are unusual or unexpected. If you are looking for synonyms for this phrase, there are several options available. The term "surprising" can be used to describe anything that is unexpected or catches you off guard. "Unusual" is another term that is often used to describe things that are not typical or standard. "Eccentric" is also a great synonym that can be used to describe people or things that are unconventional or out of the ordinary. Finally, "strange" is the most straightforward synonym which can be used to describe anything that is not easily explainable or does not fit into the norm.