Quīdam, quaedam, quoddam is the endefinite adjective, meaning a certain (sing) or some (pl). It is exactly the same as the relative pronoun, except with "dam" added to the end. Also, in the masc & fem. acc. forms, as well as in the genitive forms of all three genders, the "m" at the end changes to an "n" to make the word easier to pronounce (partial assimilation).