ETAOIN SHRDLU were the "home keys" on the Linotype typesetting machine, a mechanical monster used by almost every newspaper and magazine in the metal-type era of printing. The Linotype was an amazing contraption. Typing on the keyboard put into place a line of re-usable metal molds (matrices), each the size and shape of a single letter. Molten lead (kept liquid by a furnace built into the machine) was squirted into the matrices. The lead cooled, forming a solid "slug" representing a line of type (Line O' Type, get it?). The individual lines were then composed into pages for the press. The matrices were mechanically sorted and used over and over. The process was much faster than hand typesetting, which had been the standard.
Edited to add: ETAOIN SHRDLU were the first two vertical columns on the Linotype keyboard. When a typesetter made a typing error he'd often fill the line out by running his fingers down those columns. The bad line would be tossed (back into the lead furnace) and the line re-set. Resetting the whole line was about the only practical way of correcting errors. Sometimes the proofreader would miss one of these lines and ETAOIN SHRDLU would show up in the printed text.
I saw Linotype machines in use at the local weekly newspaper when I was a kid. Science-fiction author Fredric Brown wrote a short story "Etaoin Shrdlu" about a typesetting machine that becomes sentient. Here's an interesting video about the Linotype:
https://www.e2f.com/blog/etaoin-shrdlu-the-ghost-of-keyboards-past