Great pun. Plumbers use a plumb line which is a tool that measures a perfectly straight vertical line. Someone who is plumb crazy is a little off center. And most guys think women, especially older women, are crazy.
“Purple combines the stability of blue and the energy of red. Purple is associated with royalty. It symbolizes power, nobility, luxury, and ambition. It conveys wealth and extravagance. Purple is associated with wisdom, dignity, independence, creativity, mystery, and magic.”
There are a number of reasons why older women wear purple (including plum). Subdued purples look better on older white women than the brighter jewel tones and pastels.
Historically, 19th century women went into deep mourning (all black) for a year or two and then to white trim or gray.. After deep mourning was done young widows switched to jewel tones but older women stayed in the purples. Older women spent a lot of time in mourning because they had specific periods for spouses, parents, siblings and children.
Today there is a social group called the Red Hat Society whose “uniform” is a red hat (often trimmed in purple) and a coordinating purple outfit. They are women over 50 who “live life to the fullest” with the support and companionship of their peers.
So older women are usually considered are often considered both plum crazy and plumb crazy because we start to act more independently as our heavy responsibilities to others drop away.