Psi (Ѱ, ѱ) is a letter in the early Cyrillic alphabet, derived from the Greek letter psi (Ψ, ψ). It represents the sound /ps/, as in English naps. According to the school rules developed in the sixteenth and seventeenth centuries, such as Meletius Smotrytsky's grammar book, it was intended for use in words loaned from Greek—but it was occasionally used for writing native words, for example Ukrainian ѱы (psy, “dogs”). It was used especially in words relating to the Church, as can be seen in its continuing use in Church Slavonic.
Simovyč, V. and J.B. Rudnycky, “The History of Ukrainian Orthography”, in Kubijovyč, Volodymyr ed. (1963), Ukraine: A Concise Encyclopædia, v 1. Toronto: University of Toronto Press. ISBN0-8020-3105-6.