caster (n.2) Look up caster at Dictionary.com
"wheel and swivel attached to furniture," 1748, agent noun from cast (v.) in the old sense of "turn." Also sometimes castor.
caster (n.1) Look up caster at Dictionary.com
"person or thing that casts," late 14c. (also sometimes castor), agent noun from cast (v.). Meaning "pepper shaker, small perforated container" is from 1670s, on notion of "throwing."