From Tolkien Gateway

-lma pronominal Quenya ending meaning "our", attested (with the genitive ending -o that displaces final -a) in the word omentielmo "of our meeting". This "our" is plural inclusive.

Frodo (it seems) improperly used it in the greeting "a star shines on the hour of our meeting": since he and the Elf he was speaking to constituted only two persons, he should have used the dual inclusive instead. The correct form omentielvo occurs in some of the manuscripts of the Red Book.[1]