The term has, like so many others, changed its meaning over the centuries. Originally, it was a piece of wood inserted into the centre front of a corset or bodice to achieve the straight line.
There were pockets in the CF to receive the busk, which during the 18th century, if used at all, were often given as a token by suitors. (Who wanted their gift to be as near as possible to... you know where ...) They were long triangles when seen from the front, and triangular in cross-section, often carved, made from wood or bone.
Busks were not always used during the 18th century, and actually it seems, from surviving garments, that they were used rather rarely. During the 19th century, the meaning of the term changed to denote a device made of two strips of steel carrying eyes on one strip and nails on the other, which was used to close a corset in front before it was laced in back.