hagiology n.

before vowels hagi-, word-forming element meaning "of a saint, saintly, holy," from Greek hagios "sacred, devoted to the gods" (of things), "holy, pure" (of persons), in Ecclesiastical Greek, "a saint," which is perhaps from PIE *yag- "to worship, reverence" (source also of Greek agnos "chaste," Sanskrit yajati "reveres (a god) with sacrifices, worships," Old Persian ayadana "temple").

word-forming element indicating "branch of knowledge, science," now the usual form of -logy. Originally used c. 1800 in nonce formations (commonsensology, etc.), it gained legitimacy by influence of the proper formation in geology, mythology, etc., where the -o- is a stem vowel in the previous element.

The second element is prop[erly] -logy (-logue, etc.), the -o- belonging to the preceding element; but the accent makes the apparent element in E[nglish] to be -ology, which is hence often used as an independent word. [Century Dictionary] 

fem. proper name, mid-12c., from Old French Agnes, from Greek Hagnē "pure, chaste," fem. of hagnos "holy, sacred" (of places); "chaste, pure; guiltless, morally upright" (of persons), from PIE *yag-no-, suffixed form of root *yag- "to worship, reverence" (see hagiology).

St. Agnes, martyred 303 C.E., is patron saint of young girls, hence the folk connection of St. Agnes' Eve (Jan. 20-21) with love divinations. In Middle English, the name was frequently written phonetically as Annis, Annys. In U.S., among the top 50 names for girls born between 1887 and 1919.