无故

without cause or reason

not to have / no / none / not / to lack / un- / -less

happening / instance / reason / cause / intentional / former / old / friend / therefore / hence / (of people) to die, dead

no cause, no reason (idiom); completely uncalled for

without good cause

to have no basis in fact (idiom)

variant of 平白無故|平白无故[ping2 bai2 wu2 gu4]