What does "plain jane" mean?

A young woman or girl of average or unremarkable appearance.

What is a synonym for "plain jane"?

Common synonyms include: common, dowdies, drabs.

noun, slang

noun, slang ·Uncommon ·College level

Noun

1
A young woman or girl of average or unremarkable appearance. informal
"Charm school instructors […] can do something even for the plainest of plain Janes."
2
Something basic and ordinary. attributive, informal
"Many of the early models had both a "plain Jane" model with open sights and a Deluxe (DL) version that had an aperture rear sight and a hooded target-type front sight-and, in some cases, sling swivels."

"Charm school instructors […] can do something even for the plainest of plain Janes."