Dictionary of Spoken Russian/Russian-English/Section 19

§19. НЕКИЙ, СЕЙ

The pronominal adjective некий "some," "a kind of" is used only in writing and in bookish speech. It has some forms from a stem [njek-], some from a stem [njekoy-], and the Pn некий from a stem [njekjiy-]: Mn некий, g некоего, d некоему, i неким or некоим, l некоем; Nn некое; Fn некая, a некую, g, d, l некой or некоей; Pn некий, gl неких or некоих, and so on.

Another old-fashioned pronominal adjective is сей "this," F сия, N сие, P син, as F сию; the remaining forms are from a stem [sj-]; gs M, N сего, dls F сей, etc. This word survives in a few expressions: сегодня "today," сейчас "right away."