The term "mainframe" comes from telephone switching technology -- the
electromechanical kind from before the time of electronic telephone switches. Its
association with computers is from the earliest days of the commercial computer business
and precedes the minicomputer era by quite a bit.
As I understand it, in an electromechanical telephone switch, the "main frame"
(note the space) was the rack where the central control and switching components for the
switch were located. In a computer system, it usually applies to the cabinet where the CPU
and memory reside, perhaps including the I/O controllers, as opposed to the peripheral
components.
The term "mainframe" is still used today for enterprise-class systems (to the
horror of their marketing and advertising minions), even though the mainframe components
may be wholly contained a 1U module for a 19-inch rack.