A verb whose past form appears in the Namárië poem in the phrase ar ilyë tier undulávë lumbulë “and all paths are drowned deep in shadow” (LotR/377; RGEO/58). It is a combination of undu “down” and lav- “lick” (PE17/72). Thus, its literal meaning is “lick down” and it has various other less-literal translations such as “swallow, wash down, submerge” (PE17/72).
I suspect this verb is purely poetic and not used in ordinary speech, but if it is used outside of poetry I believe its closest meaning would be “swallow, ✱engulf”. This is because in Notes on Galadriel’s Song (NGS) from the late 1950s or early 1960s Tolkien glossed it “down-lick = swallow” (PE17/72), and similarly translated its past tense as “swallowed (lit. down-licked)” in the prose Namárië from The Road Goes Ever On of 1967 (RGEO/59). In particular, I think the gloss “drowned” in the Namárië poem from The Lord of the Rings is a loose translation.
undu adv. (and prep.?) "down, under, beneath" (UNU, VT46:20); prefixundu- "down", in undulávë "down-licked" = covered. (Nam)