Is felicity a Medieval name?
No, Felicity is a name derived from the Latin word felicitas, meaning "good luck"
There are two saints known as Felicity. Saint Felicity (Felicitas) is a legendary martyr said to have lived in Rome between about 101 and 165 CE. This information comes from a sixth-century 'Acts', which may have been the first record of St. Felicity, as her name does not appear in the well-known fourth-century Roman calendar. Another legendary Saint Felicity was said to have been a slave of Perpetua, and both died as martyrs in Carthage…
There was no character or actress named "Felicity" in The Girl was Young, a retitled Young and Innocent (1937). A full search of all TV and Movies for Alfred Hitchcock shows only one character with the name "Felicity" (Felicity Sampson) in: "The Alfred Hitchcock Hour" Day of Reckoning Season 1 episode 10 - (1962), played by Dee Hartford.