unprovoked
adj. B2 Upper Intermediate US //ˌənpɹəˈvoʊkt// UK //ʌnpɹəvˈəʊkt// un·pro·voked
adj. happening without any reason or warning. You use this to describe an action that is sudden and unexpected, especially if it is rude or violent.
adj. occurring without any prior cause or warning; lacking a logical or justifiable reason. Often used to describe aggressive or hostile behavior that appears to be spontaneous.
He was surprised by the unprovoked attack.
The dog gave an unprovoked bark at the mailman, even though the man had never done anything to it.
The diplomat was taken aback by the unprovoked criticism from the committee, which seemed to ignore the months of careful preparation that had led to the summit.
From un- + provoked.