An it girl is a young woman, often a socialite, actress or model, who becomes the person everyone is watching, copying and photographing for a season or a decade. The It in question is that elusive mix of charm and magnetism that cannot be bought or taught.
Rudyard Kipling was already using It in roughly this sense in Mrs Bathurst, a short story of 1904. The word really took off with Elinor Glyn, whose 1926 novella It turned the little pronoun into shorthand for irresistible appeal; Glyn insisted that It could be mental as well as physical. A year later Paramount made the silent film It, starring Clara Bow, who became the original It girl. Bow later confessed she was never quite sure what It meant, which may be the most accurate description of It ever offered.