The phrase is derived from a well-known adult anime (hentai) and manga series, most notably titled (姉はヤンママ授乳中). To understand why this title generates significant search traffic, it helps to break down the Japanese terminology:
Anime and manga discussion forums often have threads dedicated to "hidden gem" slice-of-life stories, where users recommend series featuring unique family dynamics [2]. 5. Why the "Yanmama" Trope is Popular searching for ane wa yanmama inall categories