出典:Wiktionary
A clipped or hypochoristic form of Joanna, from Latin Joanna, from Koine Greek Ἰωάννα (Iōánna), from Hebrew יוֹחָנָה (Yôḥānāh, literally “God is gracious”), the feminized form of יְהוֹחָנָן (Yəhōḥānān) which produced John and its many doublets. As a placeholder name, cf. similar use of John and Jack.
Doublet of Ivana, Jana, Jane, Janice, Janis, Jean, Jeanne, Jen, Joanna, Joanne, Johanna, Juana, Shavonne, Sian, Siobhan, Shane, Shaun, Shauna, and Sheena.
Joan
Joan was the usual feminine form of John in the Middle Ages. It was superseded by Jane in the 17th century, but was again very popular during the first half of the 20th century.
Joan (複数形 Joans)
Despite being a common noun, the word is still treated as a name and capitalized as such. In the 16th and 17th centuries, often with implications of plain appearance but sexual availability. In the 18th and 19th centuries, often with implication of rusticness.