Lexicalised < *an-yak 'if (it) be'; *y of *yak drops after the nasal coda (*Cyak > Cak → anak). Well grounded: Tamura (1996) records the etymology verbatim ([< an-yak]), and the same y-drop is synchronically visible (an=yakka → anakka). Shiratori (2026) places the *…n-yak series after the *nia>*ne chain shift (relative chronology).