心太

 ところてん on Japanese Wikipedia

 Tokoroten on Wikipedia

/kokorobuto/ → /kokorotei/ → /kokoroten/ → /tokoroten/

Shift from kokorobuto (see below).

The shift from buto to tei was not a regular sound shift, but rather a reanalysis of the 太 kanji, switching out the kun'yomi for the on'yomi of tai, for which tei appears to be a variant.^[1] This shift appears to have happened during the Muromachi period. Kokorotei then became kokoroten, shifting again to tokoroten by the Edo period. The ten ending is likely influenced by the ten in 天草 (tengusa, “agar”), its main ingredient.

• (Tokyo) ところてん [tòkórótéń] (Heiban – [0])^[2][3]
• IPA^(key): [to̞ko̞ɾo̞tẽ̞ɴ]

心太(ところてん) or 心太(トコロテン) • (tokoroten) 

1. a Japanese noodle-like dish made from the agar of red algae
2. Synonym of 天草 (tengusa): a red algae of the family Gelidiaceae
3. (gay slang) prostate orgasm

• 於胡(オゴ)海苔(ノリ) (ogonori)
• 寒天(かんてん) (kanten)
• 寒天(かんてん)質(しつ) (kanten-shitsu)
• 天草(テングサ) (tengusa)

Likely from Old Japanese, first attested in the Wamyō Ruijushō (938 CE).

Compound of 心(こころ) (kokoro, “heart → spirit”) + 太(ふと) (futo, “fat”). The futo changes to buto as an instance of rendaku (連濁)

心(こころ)太(ぶと) • (kokorobuto) 

1. (dated, possibly obsolete) Synonym of 天草 (tengusa): a red algae of the family Gelidiaceae
2. (dated, possibly obsolete) a Japanese noodle-like dish made from the agar of red algae such as Gelidium and Gracilaria
3. Synonym of 大根 (daikon): the Japanese radish

心(こころ)太(ぶと) • (kokorobuto) ^†-nari

1. bold, unfazed (Can we verify⁽⁺⁾ this sense?)