Example with a context (Object-Oriented Design in Java by Bill Mccarty and Stephen Gilbert (1998)):
If you remember your first programming class, this might set off a light bulb. Before the advent of structured programming, back in the days of "iron men," when "big-iron" was not merely metaphorical, computer programs were largely monolithic—they had no procedures at all. Thus, when a programmer needed to execute a piece of code in another part of the program, an unconditional branch was used; such branches were called gotos. As programs got larger, the typical path of program execution began to resemble a large web. Such code became known as spaghetti code, code that was difficult or impossible to understand and thus difficult or impossible to maintain, fix, or change.
Big iron is a slang expression that computer-savvy people use to describe those large mainframes that were used by first computer scientists during the dawn of the computer era. But who were those iron men the author is talking about?