1970-75; called floppy from the fact that the earliest disks are ‘flexible’, though the later (3.5 inch) disks have a rigid plastic case
British Dictionary definitions for floppy disks
floppy disk
noun
1.
a flexible removable magnetic disk that stores information and can be used to store data for use in a microprocessor Also called diskette, flexible disk
floppy disks in Science
floppy disk
(flŏp'pē) A flexible plastic disk coated with magnetic material and covered by a protective jacket, used for storing data. Floppy disks were once the principal storage medium for personal computers, but inexpensive hard disks and writable compact disks have greatly diminished their role.