发痒 is a verb-object compound meaning 'to itch'. As a separable verb (离合词), you can insert measure words, durations, or complements between 发 and 痒 when counting or specifying; for example, 发过一次痒 ('itched once') or 发了几天痒 ('itched for several days'). The subject is usually the body part that itches, like 手发痒 ('hands itch').