牡蠣

Probably from 蠣 (OC *mə-rat-s), with the preinitial *mə- written out as 牡 (Baxter and Sagart, 2020).

牡蠣

1. oyster
   • 2005, 5:51 from the start, in 《我的野蠻奶奶》 [Wars of In-laws]‎, episode 6, spoken by 喜塔臘鑠蘭 [Hei Tat-Lap Sheok Lan] (汪明荃 [Liza Wang]):

     呢一碗呢就係阿膠熟地煲牡蠣，滋陰補氣，養血固胎嘅。 [Cantonese, trad.]
     呢一碗呢就系阿胶熟地煲牡蛎，滋阴补气，养血固胎嘅。 [Cantonese, simp.]

     ni¹ jat¹ wun² ne¹ zau⁶ hai⁶ o¹ gaau¹ suk⁶ dei⁶ bou¹ maau⁵ lai⁶, zi¹ jam¹ bou² hei³, joeng⁵ hyut³ gu³ toi¹ ge³. [Jyutping]

     This bowl is oysters with donkey-hide gelatin and rehmannia, for enriching yin, tonifying qi, nourishing blood, and securing the foetus.

Probably a shift from either:

• 掻き (kaki, “scratching, scraping”), the 連用形 (ren'yōkei, “continuative or stem form”) of verb 掻く (kaku, “to scratch, to scrape”), from the way that oysters are harvested by scraping them off of the oyster bed
• 欠き (kaki, “breaking, chipping”), the 連用形 (ren'yōkei, “continuative or stem form”) of verb 欠く (kaku, “to break, to chip”), from the way that one must break or chip an oyster shell to get at the meat

The spelling is from Chinese 牡蠣／牡蛎 (mǔlì), as an example of jukujikun (熟字訓).

• (Tokyo) か​き [káꜜkì] (Atamadaka – [1])[1][2][3]
• IPA^(key): [ka̠kʲi]

牡蠣(かき) or 牡蠣(カキ) • (kaki) 

1. oyster (edible)

As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts (where katakana is customary), as カキ.

From Middle Chinese 牡蠣 (MC muwX ljejH).

• (Tokyo) ぼれー [bòréé] (Heiban – [0])[1]
• IPA^(key): [bo̞ɾe̞ː]

牡(ぼ)蠣(れい) • (borei) 

1. an oyster
2. roasted and powdered oyster shell, used in antacids, sedatives, and as a bird feed additive

• 真珠貝(しんじゅがい) (shinjugai, “pearl oyster”) (inedible)

• Etymology entry for 牡蠣 at Gogen Jiten (Etymology Dictionary; in Japanese): http://gogen-allguide.com/ka/kaki_kai.html
• Etymology entry for 牡蠣 at Japan Dictionary (in Japanese): http://www.nihonjiten.com/data/45551.html