A dickie is
the collar and front-placard element of a shirt, with the rest of it
simply left off. At once frou-frou and utterly practical, it added a
flourish of linen at the neck which was far more easily washed, dried
and ironed than an entire garment.
According to Wiki's excellent entry on
dickeys, the original versions were popular when sweat labor was
carried out in formal costume--for example by waiters, conductors, and
magicians. In the days before easy-care fabrics, they'd throw on a
dickie made of cloth, celluloid plastic, or even cardboard between
undershirt and tux, where it would do an excellent imitation of a
Young women began wearing dickies in subsequent decades, especially
under knit jumpers/pullovers and cardigans, to add a flash of innocent,
frilly white that framed the face in a fetching way. On occasion
designers would fashion up a deluxe dickey.