The Etymologies seem to list this word as a noun, but it is clearly the third person singular of the verb
Noldorin
lhammas
proper name. Account of Tongues, Linguistic History
lhammas
noun. account of tongues
The Lhammas
The Lhammas
blâb
verb. (he) flaps, beats
lhâf
verb. (he) licks
lhôd
verb. (he) floats
orthor
verb. (he) masters, conquers
osgar
verb. (he) cuts, amputates
síla
verb. (he) shines white
sôg
verb. (he) drinks
thia
verb. it appears
tôg
verb. (he) leads, brings
tôl
verb. (he) comes
According to WJ/301, the expression tôl acharn "vengeance comes" was later changed to tûl acharn by Tolkien
The name lhammas is Noldorin and roughly means "assortment of languages" (cf. Quenya word lambë). When Tolkien revised the history of the languages and Noldorin became Sindarin, he renamed the word as lammas (initial lh changed to l. See also: Noldorin).