aliquis

aliquis is quis with ali- bolted on the front, and only the back half declines: aliquis, aliquid, genitive alicuius, dative alicui, accusative aliquem and aliquid, ablative aliquo and aliqua. Used as an adjective rather than a pronoun, the masculine is aliqui and the neuter aliquod.

It means "someone or other", a person or thing that exists but is not being identified. The neuter aliquid takes a partitive genitive rather than an adjective agreeing with it, so "something good" is aliquid boni. The rule every Latin class chants is "after si, nisi, ne and num the ali- falls away": where you expect aliquis after one of those words you get bare quis, still meaning "anyone".