At first glance, readers unfamiliar with the word fatuous might assume that it has something to do with obesity. The term, however, derives from the Latin word fatuus, meaning “silly” or “stupid.” It may stem ultimately from a word meaning “of speech,” with the implication that a fatuous person is one notorious for saying silly or stupid things.
The adjective’s other parts-of-speech forms are the adverb fatuously and the nouns fatuity and fatuousness.
Interestingly, many synonyms for fatuous are variations of compounds ending with the root -headed: Some, like airheaded, suggest a lack of brains; others, like lunkheaded, imply that a fatuous person’s head is solid or thick. An etymologically unrelated but synonymous word that nearly rhymes with fatuous is vacuous, associated with the first class of words ending in -headed; the word is related to vacuum, suggesting a void within a person’s skull.
The term infatuation (and its verb form, infatuate) is based on the same root word as fatuous: It refers to a foolish passion. Usually, that passion is love (or what the infatuated fool self-deceptively believes is love), but it can also refer to an unreasoning adherence to a belief or a cause or a platonic devotion to a person.
Also related is “ignis fatuus”—literally, “foolish fire.” This Latin name for a will o’ the wisp, or light generated by combustion of gas from decomposing organic matter, is also used figuratively to refer to a deceptive goal or hope—a beacon of folly. (Mirage, the term for an optical illusion resembling water or other shining material or objects, has a similar figurative connotation.)
Ignis is the origin of ignition, referring to the act of causing something to start burning, and is related to igneous, an adjective pertaining to volcanic rock.
“Will o’ the wisp,” meanwhile, consists of the abbreviated version of the name William (previously sometimes used, like Jack or Bud, to refer to a generic person), as well as a contraction of of, plus the and wisp, meaning “a thin strand or streak.” In its figurative sense, “will o’ the wisp” suggests a frail thread of longing a fatuous person clings to.