(n.) the opposite of megalomania—an extreme tendency to belittle yourself or trivialize your achievements
When we call someone a megalomaniac today, we tend to mean that they are grossly self-obsessed, driven by grandiose schemes, or fixed on achieving ever more power or dominance.
The term has been tossed around this way for over a century: even as early as 1897, the British Prime Minister Lord Salisbury memorably accused his long-time sparring partner William Gladstone of suffering from “a common intellectual complaint,” better known as, ”megalomania—the passion for big things, simply because they are big.”
When it first appeared in the language in the 1880s, however, megalomania was a strict psychological term: a delusion identified by Victorian physicians in patients who would grossly overestimate themselves, their strength or capabilities, or else imagine that they had somehow developed godlike or divinely appointed powers. That sense steadily weakened as the word became more familiar—and, as it did, so too did its seldom-used opposite.
Micromania likewise began as the name of a curious delusion when it first appeared in the language in the late 1800s: originally, micromaniacal patients would believe that their body (or some part of it) had somehow become vastly reduced in size, leading to noticeable problems of perception and awareness.
But as the meaning of megalomania broadened, so too did that of micromania. By the turn of the century it was being used to refer to a pathological self-belittlement—an automatic, compulsive tendency to shun praise, to trivialize successes or achievements, and to figuratively hide your light under a bushel.