Lewis & Short

ădoptātīcĭus (not -tītĭus), a, um, adj. [adopto], adopted, received in the place of a child; only in Plaut., Poen. 5, 2, 85: Demarcho item ipse fuit adoptaticius, ib. 100.
Acc. to Festus, it signifies the son of one who is adopted: ex adoptato filio natus, p. 29 Müll.