Hickey is an Americanism that appeared in 1909 to mean a dingus, doodad, thingamabob, thingamajig, thingummy, whatchamacallit, whatnot, or whatsit. For this role it was deemed a bit bland, and so by 1914 it had been expanded to doohickey, a form that has proven more popular and lasting.
These days, a hickey is the name for a specific tool, a sort of wrench that is used for bending pipes; it consists of a open, fixed jaw to encircle the pipe, with the lower part protruding forward a bit and a long handle to act as a lever. When the jaw is placed around the pipe and the handle pulled back, the lower bit pulls up, the upper bit pushes back and down, and the pipe bends. These are sometimes known more formally in modern hardware stores as a conduit bender.
Hickies, in the plural, is also used to refer to small imperfections when printing.
In 1915 the word was used to informally refer to pimples and other skin blemishes; in the early 1930s the corrupt youth of the 'Rock and Roll' era co-opted it to refer to marks left on the skin from biting or sucking when making out.