Where does “Lēthē” come from?

Lēthē (Latin) comes from Ancient Greek Λήθη, from Proto-Hellenic lā́tʰā, from Proto-Indo-European léh₂dʰeh₂, from Proto-Indo-European leh₂- — to bark, howl; to be concealed.

Lēthē (Latin): the river Lethe, the river of oblivion