Tantalus was the son of Zeus and the Titaness Pluto. His grotesque fate of having to stand up to his chin in water with fruit overhead that receded whenever he tried to reach it has given us the word 'tantalise'. There seems little doubt that his name is based on the talas or 'suffering' root that we considered for both Talos and Talthybius and saw in the names of Atlas and Telamon.
Plutarch derived it from the superlative of talas - talantatos, 'mos wretched'. But talanton means 'balance', 'pair of scales', and this may refer to his riches for which he was famous, so that he is a kind of personified greed: having much, he ever wishes for more.
There was even a Greek saying about the 'talents of Tantalus' (Tantalou talanta) which came to mean more or less the same as the 'riches of Croesus'. (In this respect, his mother's 'rich' name is significant.) A somewhat more complex theory, although on the same lines, sees his name as meaning 'flourishing', and as being a reduplication (as for Sisyphus) of thallo, 'to flourish'. This again refers to his wealth and his insatiable desire for more.
The reduplication would have caused an original name such as 'Thalthalos' to become Tantalus, with 'th' alternating with 't', and Τ with 'n', as they sometimes did (for example, Ionic aythis, 'again', 'back' for aytis, and Attic litron for Doric nitron, 'soda').