The noun arma, -orum is neuter plural always according to my book. I thought neuter nouns ended in -a. What is the -orum then? Thanks.

Answer 1

The ending -orum is the genitive form. In the case of this word, since it doesn't have any singular forms, you give the nominative and genitive of the plural.

Answer 2

In the link I gave you above, scroll down to the section called Neuter Nouns ending in -um. Look at the plural column; ignore the singular for this particular word.