The tendency of manufacturers (or, by extension, other designers) to come up with products that don't fit with the old stuff, thereby making you buy either all new stuff or expensive interface devices.
The term probably came into prominence with the appearance of the DEC KL-10, none of whose connectors matched anything else. The KL-10 Massbus connector was actually *patented* by DEC, who reputedly refused to licence the design, thus effectively locking out competition for the lucrative Massbus peripherals market. This policy was a source of frustration for the owners of dying, obsolescent disk and tape drives.
A related phenomenon is the invention of new screw heads so that only Designated Persons, possessing the magic screwdrivers, can remove covers and make repairs or install options. Older Apple Macintoshes took this one step further, requiring not only a hex wrench but a specialised case-cracking tool to open the box.
With the advent of more open-systems computing this term has fallen somewhat into disuse.
Compare backward combatability.