During the Chinese Civil War, the Nationalists and Communists each called the other 匪 - the Nationalists said 共匪 (Communist bandits) and the Communists said 蒋匪 (Chiang's bandits).
The right-opening box wraps around 非 from the left. Originally a basket or chest, here it suggests something concealed inside - a bandit hiding in cover. As the indexing radical it groups 匪 with 匠, 区 and 医 in the framed-content family.
Sits inside the box and lends the sound, drifting from fēi to fěi. Its meaning of wrong / not also pulls double duty: a bandit is essentially someone hidden away who is doing wrong - the box framing the wrongness perfectly.
