Someone described as fastidious is extremely or overly careful about how they do something. Fastidious may also describe someone who is difficult to please, or someone who always wants to be clean, neat, etc.

// Our parents taught us to be fastidious in keeping our rooms clean, making sure to dust every surface and sweep out every corner.

// He's a fastidious dresser whose fashion choices seem to anticipate the newest trends.

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"Becoming Led Zeppelin, filmmaker Bernard MacMahon's new documentary about the band, certainly succeeds at taking Led Zeppelin seriously, in ways that might disappoint some viewers but that I found both compelling and refreshing. Becoming Led Zeppelin doesn't hide that it's an authorized biopic … but the film is so fastidious and detail-oriented that it never feels like hagiography." — Jack Hamilton, Slate, 11 Feb. 2025

If you presume that the adjective fastidious bears some relation to fast, not so fast. Fastidious comes from Latin fastidium, meaning "aversion" or "disgust." Fastidium is believed to be a combination of fastus, meaning "arrogance," and taedium, "irksomeness" or "disgust." (Taedium is also the source of tedium and tedious.) In keeping with its Latin roots, fastidious once meant "haughty," "disgusting," and "disagreeable," but the word is now most often applied to people who are very meticulous or overly difficult to please, or to work which reflects a demanding or precise attitude. Our own fastidiousness requires us to point out that the familiar adjective fast comes not from Latin, but from Old English.