Caldwell in his 'History of Thinnevelly' book, says it might have come from Tambapaṇṇī, used by Prince Vijaya since the river called as 'porunai ( #பொருநை) in #Tamil.

' #Tāmraparṇī ( #தாமிரபரணி)' is the Sanskritised version ' #Tambapaṇṇī' as Adam mentioned.

1/ https://twitter.com/JLC1956/status/1338124982564630531

He also asserts the same claim of Prince Vijaya being married to a daughter of #Pāṇdya, this new relationship could have brought the name to #Korkai ( #கொற்கை).

#Tāmraparṇī ( #தாமிரபரணி) = Tāmra (red, copper) + parṇī (leaves, tree). He says it meant 'a tree with red leaves'

2/

Apparently, it was called 'Tambapaṇṇī' by Buddhists and inscribed as the same in the Girnar edict of #Asoka.

One of the #Theravada #Buddhist sect in SL today called ' #Tāmbraparṇiya'.

I suspect #Tambapaṇṇī could have been #Pāli or #Prakrit.

3/

The Ancient Greeks called SL as ' #Taprobánē', but apparently called the river 'solen'. He doesn't provide great detail about why, since Greek has the word 'solen' meaning 'channel, gutter, pipe'.

He notes that by the mouth of the river there are plenty of chanks.

3/

And, suspect that the Greeks called the river by #chank (a large sea conch).

Oh, wait, #chank -> that sounds like 'caṅku ( #சங்கு)' (another English spelling @JLC1956).

Greeks speak of 'solen' born from ' #Bettigo', which aligns with 'Potikai ( #பொதிகை), #Malaya of #Purāṇa.

4/

Varāhapurāṇa chap 85 says, #Tāmraparṇī a river originating from #Malaya, a holy mountain ( #kulaparvata) in #Bhārata, ( #janapada) where Āryas and Mlecchas dwell who drink water from these rivers.

#Mahābhārata, #Rāmāyaṇa, and #Raghuvaṃśa also mention of #Tāmraparṇī

5/

#Śloka 14 of #VanaParva in #Rāmāyaṇa says, #Tāmraparṇī a holy river of Dakṣiṇa Kerala. Once, the #Deva-s did penance on the banks of this river to obtain salvation.

#Mahābhārata (cf. III.86.11) refers to Tāmraparṇī as the name of a #tīrtha.

#DDSA gives this.

6/

smaraṇāt darśanāt dhyānāt snānāt pānādapi dhruvam | karmavicchedinī sarvajantūnāṃ mokṣhadāyinī - tāmraparṇī māhātmyam

The rivers #Tāmraparnī and #Vegavatī accord Brahmaloka - #Śivapurāṇa 1.12

7/

From the citations above, one could safely say that #Tāmraparnī was a #Sanskrit word used to name the river.

I am not sure if #Tambapaṇṇī is the corruption of the Skt name based on #Purāṇas or vice versa.

8/

In #Tamil lit,

தண் பொருநைப் புனல் பாயும்
விண் பொருபுகழ், விறல்வஞ்சிப் - புற 11

Here #தண்பொருநை indicates Amarāvati ( #அமராவதி) based on this poem is about a #Cēra king and the place is Vañci ( #வஞ்சி).

9/

porunaittuṟaivaṉ ( #பொருநைத்துறைவன்) is also used to call #Cēran.

Similarly, #Pāṇdyan also mentioned as porunaittuṟaivaṉ.

10/

..நாரரிநறவின் மகிழ் நொடைக்கூட்டும்
பேரிசைக் கொற்கைப் பொருநன் வேண்வேல்
கடும்பகட்டியானை நெடுந்தேர்ச் செழியன் - அக 296

Here #Ceḻiyaṉ, named as #பொருநன்.

..இருங்கோ வேண்மான் இயல்தேர்ப் பொருநன் என்று எழுவர் - அக 36

Looks like Vēḷir ( #வேளிர்) chieftain also called as #பொருநன்.

So, I suspect #பொருநன் could have been a common name for kings/chieftains.

12/

And, most Sangam poems refer #தண்பொருநை or #தண்ணான்பொருநை to #அமராவதி near #கரூர்.

தன்ஆன் பொருநை மணலினும் பலவே - அக 20
கல்லென் பொருநை மணலினும், ஆங்கண் - அக 387
தண் பொருநைப் புனல் பாயும் - புற 11
தண்ணான் பொருநை வெண்மணல் சிதையக் - புற 36

I assert #தண்பொருநை is #அமராவதி.

13/