Big iron, as the hacker's dictionary the Jargon File

Jargon File

The Jargon File is a glossary of computer programmer slang. The original Jargon File was a collection of terms from technical cultures such as the MIT AI Lab, the Stanford AI Lab and others of the old ARPANET AI/LISP/PDP-10 communities, including Bolt, Beranek and Newman, Carnegie Mellon...

 defines it, "refers to large, expensive, ultra-fast computers. It is used generally for number crunching supercomputer

Supercomputer

A supercomputer is a computer at the frontline of current processing capacity, particularly speed of calculation.Supercomputers are used for highly calculation-intensive tasks such as problems including quantum physics, weather forecasting, climate research, molecular modeling A supercomputer is a...

s such as Cray

Cray

Cray Inc. is an American supercomputer manufacturer based in Seattle, Washington. The company's predecessor, Cray Research, Inc. , was founded in 1972 by computer designer Seymour Cray. Seymour Cray went on to form the spin-off Cray Computer Corporation , in 1989, which went bankrupt in 1995,...

s, but can include more conventional big commercial IBM mainframes

Mainframe computer

Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

".

The term bears a slightly humorous, but respectful, connotation.

The term is often used in reference to IBM mainframe

IBM mainframe

IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...

s, often when discussing their comeback/survival after the assault of lower cost Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 systems. More recently the term is also applied to powerful computer servers and computer ranches, whose steel racks naturally invoke the same association.

The expression may be compared with the slang

Slang

Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 expression for heavy handgun

Handgun

A handgun is a firearm designed to be held and operated by one hand. This characteristic differentiates handguns as a general class of firearms from long guns such as rifles and shotguns ....

s, derived from the slang "iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

" for a handgun ("shooting iron"), as exemplified by the classical country music

Country music

Country music is a popular American musical style that began in the rural Southern United States in the 1920s. It takes its roots from Western cowboy and folk music...

 ballad Big Iron

Big Iron

"Big Iron" is a country ballad by Marty Robbins, originally released as an album track on Gunfighter Ballads and Trail Songs in September 1959, then as a single in February 1960....

by Marty Robbins

Marty Robbins

Martin David Robinson , known professionally as Marty Robbins, was an American singer, songwriter, and multi-instrumentalist...

about "the ranger with the big iron on his hip".