A keyboarder is a person whose job is to type information into a computer — names, numbers, old paper files, someone else’s words. They sit at a keyboard all day and put the data in quickly and correctly. It is an office job, not a music job, and not a programming job. The word is a bit old and a bit dry; many workplaces now say data-entry clerk, or just say that someone types the records. You still meet keyboarder in job lists and in older office writing. It is formed in a very simple way: the keyboard plus -er, a person who uses that thing. A keyboardist, by contrast, plays a piano-style instrument in a band. Same kind of keys, very different lives. Nobody uses keyboarder as slang, and it is not rude; it is just a work word that has become less common as almost everybody types.