It used to mean someone who literally destroyed icons— as in statues and things that folks would worship.

Nowadays it mostly refers to “A person who attacks cherished beliefs or institutions”. [1]

It connotes that someone isn’t afraid to loudly voice a (potentially) minority or unpopular opinion. See also ‘shit-stirrer’, ‘skeptic’.

It’s also in the title of a classic Simpsons episode. [2]

[1] https://www.lexico.com/en/definition/iconoclast

[2] https://en.wikipedia.org/wiki/Lisa_the_Iconoclast

Attacking icons; the classic iconoclasts wanted to destroy sacred church imagery as a distraction from the proper worship of god.

In O'Rourke's case it meant that while saying that he was a republican and having a go at cyclists and fat women and other "safe" targets, he was also happy to rip into Reagan for supporting Marcos against the peaceful Yellow Revolution, or his seminal essay on the war on drugs, "The Whiffle Ball Life", where he pointed out that the people selling the war could always be sure that they or their kids would never suffer any serious consequence for their own drug use, but that it would be borne by black teens and young men.

He was a butcher of sacred cows.