The witness, according to the news story, said the robbers were “plum crazy.” Not unless they were robbing a green grocer. (It was a McDonald’s.)
A “plum” is a fruit, usually of a deep purple color, also called “plum.” When dried, “plums” used to be known as “prunes” until prunes got a marketing department and became known as “dried plums.”
“Plum” as an adjective means “desirable,” as in “a plum job.” The Dictionary of American Slang says that usage arose around 1825, and may be related to Little Jack Horner and how good and lucky he was to pull “plums” out of pies. By then, the British were already considering something good to be “plummy.”
Journalists occasionally write that someone got a “plumb job.” That could be one of two things: A job that requires the employee to stand perfectly straight or a job where the employee measures things along perfectly straight lines—for “plumb” is something perfectly vertical or in line. Builders want to be sure their lines are straight or “plumb.”
Because “plumb” means truly, completely, entirely straight, the slang dictionary says, about 1748 someone was called “plumb silly.” From there came “plumb crazy” (or its twin, “plumb loco”), “plumb tired,” and other such straightforward expressions.
And yes, some “plumbers” have both “plumb jobs” and “plum jobs.” Lucky them.