Reference to the 4.6L and 5.4L V8 engines that Ford used on 96 and newer Ford Mustangs and other vehicles.
Short for "modular motor". A "modular" motor is one that shares many parts with other motors/engines of differing displacement and cylinders.
For example, Ford's 2.5L and 3.0L Duratec V6 engines are also "modular motors" (although the term "mod motor" is usually only used for the V8 engines) and share parts with the 4.6 and 5.4L V8 engines.
The mod motors are nice, but they cost a lot more to modify than the good old 5.0 engine. They're also harder to work on.