fem. proper name, from French Jeanne, Old French Jehane, from Medieval Latin Johanna (see John). As a generic name for "girl, girlfriend" it is attested from 1906 in U.S. slang. Never a top-10 list name for girls born in the U.S., it ranked in the top 50 from 1931 to 1956. It may owe its "everywoman" reputation rather to its association with the popular boy's name John.
word-forming element, originally a diminutive suffix but not now always felt as one, Middle English, from Old French -et (fem. -ete; Modern French -et, -ette), from Vulgar Latin *-ittum/*-itta (source also of Spanish -eto/-eta, Italian -etto/-etta), of unknown origin. The French forms are reduced to -et in English, but later borrowings of French words in -ette tend to keep that ending.
c. 1300, "vessel for cooking," from stew (v.). Later "heated room," especially for bathing (late 14c.). The meaning "stewed meat with vegetables" is first recorded 1756. The obsolete slang meaning "brothel" (mid-14c., usually plural, stews) is from a parallel sense of "public bath house" (mid-14c.), carried over from Old French estuve "bath, bath house; bawdy house," reflecting the reputation of medieval bath houses.
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<a href="https://www.etymonline.com/word/Janet">Etymology of Janet by etymonline</a>
Harper, D. (n.d.). Etymology of Janet. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Janet