蜿蜿長蛇 is built the way a yoji-jukugo (four-character idiom) is built, even though it isn't formally registered as one — two doubled/paired two-character blocks stacked together, each reinforcing the same image from a different angle. 蜿蜿 is the rarer half: a reduplicated classical word that most modern readers will not recognize without furigana, historically describing anything that winds on at length — a river's course, a mountain range's silhouette, or literally a snake's path. 長蛇, by contrast, is a word every Japanese reader already knows, just not usually in a combat context — its overwhelmingly dominant modern use is 長蛇の列, "a long line," the phrase for a queue outside a popular restaurant or a ticket booth. Gotouge is reclaiming a word most readers associate with mundane waiting-in-line imagery and returning it to its literal snake meaning for a Hashira's finishing technique — a small, sly joke in the kanji that the English translation has no way to carry over.
The technique's visual staging matches the compounding of the name: rather than a single strike, the panel shows Obanai's blade tracing one unbroken sinuous path through a whole cluster of demons, exactly the "long, winding, extended" image 蜿蜿 and 長蛇 both independently gesture at. Doubling up two near-synonymous winding-words for one finishing move is a naming strategy unique to this form within Serpent Breathing — Forms One and Two both use a single anatomical or descriptive term (委蛇, 狭頭), while Form Five stacks two, suggesting an escalation in scale that matches its higher number and its use as a crowd-clearing technique rather than a single-target strike.
On the corpus itself: the automated OCR misreads the first two characters as 婉婉 (using a different but visually similar kanji, 婉, meaning "graceful/beautiful," rather than 蜿, "to wind") — an understandable error given how close the two characters look at manga resolution. Direct visual inspection of the printed page confirms the correct kanji is 蜿蜿, consistent with the snake-anatomy naming pattern across the rest of Serpent Breathing; this entry displays the corrected reading while preserving the corpus's literal (mis-OCR'd) text in the byte-exact source citation below, per this dictionary's verification standard.