Let us travel back to those thrilling days of feudalism, when lords were lords and everyone else paid high taxes to the lords. (OK, maybe not so long ago.)
Back then, land was owned only by the very high up (think royalty), who would in effect grant a lease on it in gratitude for services rendered: plundering, conquering, or running amok upon an enemy, for example. That land was held in “fee,” and it came complete with whatever forests, streams, deer, villages, or serfs happened to be there at the time. The man who was granted the “fee”—and it was only men—didn’t own the land, but collected goods or taxes from the people upon it. He could pass the land on to his son, as long as all stayed in the good graces of whoever had granted it.
These grants were also called “fiefs,” and for hundreds of years the term was sufficient to refer both to the grant and to the land itself. (“Fief” is pronounced like “fee” with an “f” at the end, though it’s sometimes heard as “fie” with an “f,” rhyming with the flute.)
Then someone apparently decided that English could use another word, and, perhaps to make it sound more stately, added the suffix “dom.” The Oxford English Dictionary traces the first use of “fiefdom” to 1814.
While some countries still do have a form of “fief,” the words “fief” and “fiefdom” now are used mostly to mean a person’s sphere of influence—one news report referred to school principals as “kings of their fiefdom,” something of a contradiction, but you get the point. Lately, the realms of the Treasury and Energy Departments have been referred to as “fiefdoms.”
One way that language changes is through the invention of new words that mean the same as words that already exist, often by adding a prefix or suffix—we’re already discussed the uselessness of “irregardless,” not that it has slowed the spread of that nonstandard usage. “Fiefdom” is another in the group of words that Garner’s Modern American Usage calls “needless variants,” which “should be dropped from the language.”
“Fiefdom” now is far more common that “fief,” probably because the similarity to “kingdom” reminds one of the implied importance of the person who has a “fiefdom.” Most dictionaries include it, many with the sole definition as “fief,” so needless though it may be, it seems to have a long-term lease.