From Proto-Vietic*vɔːk. This appears to be an "internal borrowing" from the Nghệ An/Hà Tĩnh dialects which are known for retaing the old rhymes /*ɔːk/, /*ɔːŋ/, /*oːk/, /*oːŋ/, since these rhymes have evolved into diphthongized forms in most other dialects.