It is unknown how the Min forms relate to forms in other dialects. See this article for a discussion of the Min Nan etymon. For similar nasalisations in Min, compare 耳 (ěr).
Colloquial words in different Min dialects show considerable variation – most have an n- initial, but finals and tones differ greatly (although the tone is never a rising tone). Some propose that this is substrate influence, passed on from the maternal Baiyue lineages since the intermarriages between southward-migrating Han Chinese and native non-Han women. Other southern dialects also show remnants of this native word: Hakka [Meixian] nɛn5, Cantonese [Guangzhou] nin1 (姩 or 𢆡). Compare Thaiนม(nom, “breast; breastmilk”), Zhuangnoemz(“breast; breastmilk”).
The long-recognised Sino-Tibetan etymon in Chinese is 乳 (OC *njoʔ); see there for more.