The 11th phrase of the prose Namárië. Tolkien altered the text from the poetic version as follows:
> ar ilyë tier undulávë lumbulë >> ar lumbulë undulávë ilyë tier
Tolkien simply swapped the subject lumbulë “(heavy) shadow” with the object ilyë tier “all roads” to be consistent with the usual Quenya subject-verb-object word order.
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.