I don't mean to mock you or anyone for that matter, but still this acrosstic came to me during my day job as a software test engineer: A mock gets its name from the mockingbird, who lays its eggs in the nests of other bird species, trying to fool the other bird into hatching its young. In software engineering a mock is similar: it gets written to imitate another piece of software like for instance an external API or database, and allows for more control over the external parties in the system.
This allows us to test a piece of software in relative isolation, in order to make sure that one unit works before we hook it up to other systems for integration testing.