To pass something is to move beyond it or go by it. A person can pass away if he or she dies. When something happens in passing, it happens without intent, or offhandedly.
Passing as an adjective can describe something has moving past something else (e.g., a passing car) or describe something as happening for a small moment (e.g., a passing phase).
However, when passing is used as an adverb, to describe another verb, it means to a great degree, or to a large extent. So take care to note whether passing is meant as an adjective or an adverb.
This adverbial sense is how the word is meant in the idiom passing strange.
The phrase was coined by William Shakespeare in Othello when the title character describes something as “strange, ’twas passing strange” (i.e., very, very strange).