What does "boy" mean here?

Recently, in a magazine, I read the following line:

It's been an eventful year and boy, has it flown past!

My question is, what does the word "boy" mean here? Is it an idiom? What is its usage?

Boy in this usage is an interjection that is an exclamation of surprise, wonder, contempt, etc. In this particular sentence, it's just emphasizing how quickly the year has flown by.

You hear it pretty frequently in the "Oh Boy!" which used earnestly usually indicates excitement about something (or when used sarcastically, expresses dread).

wow, golly or gee whiz are similar expressions you might hear in these contexts as well. crikey fills the same function but is limited to UK/Australian English.

It's used to express surprise.

Other examples: Boy, was I wrong. Boy, was it fun.

Boy, was it hard.

The above sentence conveys that you were surprised to see it was hard when you expected not to be.

A less common usage is "man."

"Man, this is great."

It is a way of addressing no one in particular.