This adjective in the feminine nominative singular presupposes a noun such as xw/ra, territory; and the entry is probably generated by Polybius 2.21.7, th\n *pikenti/nhn prosagoreuome/nhn xw/ran. See also, for the nominative itself, Strabo 5.4.2 & 10, and Stephanus of Byzantium s.v. Pikentia.
For Picenum, near Umbria in central Italy, see also pi 1593 and Barrington Atlas map 42 grids E2-F3. (The coastal ager Picentinus in S Italy, between Salernum and Paestum, is different.)