an unusual use of the word justify
Every time you use your word processor you use justify in exactly that sense.
The standard format is "left-justified:" all lined up at the left margin. You can make it all line up at the right margin instead if you want to (right-justified), or center every line (center-justified), by clicking on a "button" on a menu bar, or perhaps by the older method of highlighting some text and then typing control-R, control-L, or control-E (as in cEnter).
When linotype machines were in use, or even earlier when type was set by hand (anyone remember the California job case?), you had to put in spacers to make all the lines come out the same length. There would be spacers between words or letters, and also between lines and paragraphs, for horizontal and vertical justification, so that the type would one rectangular block that could be set into the printing press. See our previous discussion of "coigns," by whichever spelling!
Actually we still do this, even with computerized typesetting. That's what's responsible for the all-too-frequent funny-looking appearance in your daily newspaper when they had to use a lot of spacers for long words in short lines.
