Achilles was dipped in it as an infant (4)

I believe the answer is:

styx

(Other definitions for styx that I've seen before include "Underworld river; sounds like remote area" , "Charon's 26 carried dead souls across this river" , "One of the underworld rivers" , "Underworld river with ferryman Charon" , "River around Hades" .)

I've seen this clue in The Wall Street Journal.

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