pumping iron

Robert B.Parkerの "Valediction"を読んでいます。
The anger had enlarged and was working its way up from the pit of my stomach, spreading along my back and shoulders and down my arms. I could feel my face getting hot. I was careful with my voice, easing it out so it was steady.
"This is different business," I said, "from pumping iron. It's a business I'm almost sure to be better at than you are. Don't make a mistake," The muscles in my neck and shoulders were starting to bunch on their own. My whole upper body was tense.

"pumping iron" の意味を調べます。
・Collins Dictionary: to exercise with weights; do body-building exercises: Now she spends her days pumping iron to get the biggest thighs in women's cycling.

・Wikctionary: (weightlifting, informal) The activity of weightlifting, especially for the purposes of bodybuilding.: He's just another one of Leo's thugs from the east side. Too much pumping iron jarred his brain loose!

"pumping iron" の由来がWikipediaに書かれていたので抜粋して引用します。
Pumping Iron is a 1977 American docudrama about the world of professional bodybuilding, with a focus on the 1975 IFBB Mr. Universe and 1975 Mr. Olympia competitions.
In 1975, bodybuilders are preparing for the upcoming Mr. Universe amateur competition and Mr. Olympia professional competition in Pretoria, South Africa.

...
The film then switches focus to the rivalry between Arnold Schwarzenegger and Lou Ferrigno, professional bodybuilders competing for the title of Mr. Olympia. Schwarzenegger, at this point a ten-year veteran of bodybuilding, has won Mr. Olympia for five consecutive years and intends to retire after a final competition.
シュワルツェネッガーがボディビルダーとして絶頂期の時の映画ですね。