Clara Bow "The Original It Girl"1905-1965
The term was first introduced by the screen writer Elinor Glynn to describe the actress Clara Bow starring in the silent Hollywood movie "It" in 1927. Glynn's exact definition of the term is:
"IT" is that quality possessed by some which draws all others with its magnetic force. With "IT" you win all men if you are a woman—all women if you are a man. "IT" can be a quality of the mind as well as a physical attraction.
Self-confidence and indifference whether you are pleasing or not—and something in you that gives the impression that you are not at all cold. That's "IT".
As a subtext, the movie also portrays the "it girl" as a mix of ingenuity, femme fatal look and material girl image. By contrast, her rival is equally young and comely, and even rich, blonde and well-bred to boot, but she simply hasn't got "it".
AND NOW...

Photo by Steven Klein , W Magazine August 2009
.....Lara Stone appearing as the Fashion's IT Girl on the cover of W Magazine. The magazine says:
"... There is nothing girlish about Stone. Nor is there anything boyish; Klein calls her “the girl with the X-rated lips.” She’s neither coquette nor vamp; her seduction lies in her womanliness—the breasts, the hips—and perhaps also in the palpable ambivalence Stone feels toward the industry that she finds herself at the top of at the moment.
...Bruce Weber, who has worked with Stone several times since her Givenchy re-debut, says, “To me, Lara is part Marlon Brando, part Thelonious Monk and part Robert Mitchum. She’s big, bad and beautiful.”
“She had this sexual awareness,” the photographers Mert Alas and Marcus Piggott recall via e-mail of meeting Stone two years ago, on a shoot for this magazine. “Normally when we are shooting, we are looking to create a character, and the great thing about Lara is that she is naturally already such a strong character herself [that] she inspires us to take a better picture. " "
OK, she is very pretty and femme fatale; but do you buy all this? To me, "this" girl just reminds me of another real IT girl, Brigitte Bardot, rather than Thelonious Monk... just another promotion of a new girl to the fashion media ... Still, I like the combination of this Chanel dress with the BB look...
( "If I go to a restaurant, other people stare. The meal is ruined." BB.)
For more curious from Wikipedia:
.... In fashion the Bardot neckline (a wide open neck that exposes both shoulders) is named after her. Bardot popularized this style which is especially used for knitted sweaters or jumpers although it is also used for other tops and dresses.
Bardot is recognised for popularizing bikini swimwear in early films such as Manina (Woman without a Veil, 1952), in her appearances at Cannes and in many photo shoots.
Bardot also brought into fashion the choucroute ("Sauerkraut") hairstyle (a sort of beehive hair style) and gingham clothes after wearing a checkered pink dress, designed by Jacques Esterel, at her wedding to Charrier. She was the subject for an Andy Warhol painting.