"itch for doing something," 1560s, from Latinized form of Greek kakoēthēs "ill-habit, wickedness, itch for doing (something)," from kakos "bad" (from PIE root *kakka- "to defecate") + ēthē- "disposition, character" (see ethos). Most famously, in Juvenal's insanabile scribendi cacoethes "incurable passion for writing."
1846, "capable of being declared as true," from certify + -able. Meaning "so deranged as to be certifiably insane" is recorded from 1870, from certify in the specific sense "officially declare (someone) to be insane" (1822). The certification was done by local officials, later medical officers, and often included a statement as to whether the person was harmless or dangerous, curable or incurable.