不知火 (shiranui) is not a word Koyoharu Gotouge invented for this technique — it already existed in Japanese as the name of a real optical phenomenon: mysterious lights that sailors historically reported flickering across the Yatsushiro Sea and Ariake Sea at night, long attributed to foxfire, dragon fire, or the ghosts of drowned fishermen before science identified atmospheric refraction as the likely cause. The word was distinctive enough that the Imperial Japanese Navy later used it as a warship name (an IJN destroyer, among others). VIZ's "UNKNOWING FIRE" is a genuinely interesting middle path: rather than leave 不知火 as an untranslated proper noun (the way many fan translations and dub subtitles do), it parses the two halves literally — 不知 (not known) + 火 (fire) — and lands on a phrase that personifies the fire as something that "doesn't know," which is subtly different from "fire that is unknown." It's a translation that shows its work without fully explaining the reference underneath it.
That same callout also isn't printed consistently. On the Mugen Train (Vol. 7, p. 43), used against Enmu's puppet controlling the train, VIZ hyphenates it: "UN-KNOWING FIRE!" Days later in the same arc, fighting Akaza (Vol. 8, p. 39), the identical Japanese renders as "UNKNOWING FIRE!" with no hyphen. Neither is wrong, and both are legible, but it's the same small inconsistency this dictionary has already flagged in Thunder Breathing's "GOD-SPEED"/"GODSPEED" — evidence that VIZ's lettering choices are made panel-by-panel, not locked to a style guide that persists across a single fight.
The corpus is also careful to head off a very predictable reader mistake. Earlier in Volume 7 (p. 10), before Rengoku ever appears on-page, a supporting character explicitly warns Tanjiro: "THERE ISN'T ANY SUCH THING AS FIRE BREATHING, BUT THERE IS FLAME BREATHING" — 「火の呼吸」はありませんが「炎の呼吸」はあります. 火 (hi) is raw, generic fire — the word in 火事 (kaji, "a fire/blaze") or 火事場 (a fire scene); 炎 (honoo) is specifically the visible tongue of flame, the roaring, dramatic, alive-looking part of a fire, which is exactly the register Rengoku's whole flamboyant fighting persona lives in. The manga is, in effect, pre-correcting its own audience's most likely slip before the Flame Hashira even shows up.
There's a quieter irony buried in the same Hashira-meeting scene. Rengoku dismisses further questions about Tanjiro's newly revealed Hinokami Kagura with "FLAME BREATHING HAS A LONG HISTORY!" (炎の呼吸は歴史が古い, Vol. 7, p. 35) — asserting his own style's seniority to shut down the topic. The manga later reveals Hinokami Kagura is in fact the far older, foundational dance Flame Breathing itself was adapted from — a fact Rengoku never learns before his death. His proudest claim about his own lineage is, unknowingly, backwards.