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).
The phrase is more popular outside the United States, where it’s popularity has declined since its peak in the 1800s.
Examples
Today Kublalsingh marks 100 days of his second hunger strike during which he claims not to have consumed any food or water. “He seems to be a Christmas miracle…it is passing strange and it would be nice if he could release his medical records then the Guinness World Book of Record would be proven wrong and we would have a world record holder here in Trinidad and Tobago,” said Khan. [Trinidad Express Newspapers]
I find it passing strange that the season of giving focuses so much on what can be bought rather than what can be shared. [The Telegram]