Found in the Yuan play 《生金阁》 act 3: '怎么不由分说,便将我飞拳走踢只是打' — 'why, without giving me a chance to explain, do you just rain kicks and punches?' Colloquial Yuan-drama vocabulary that fossilized into standard written use.
grammar
Functions adverbially — usually followed directly by a verb phrase (不由分说 + VP). Common with 把-constructions: 不由分说把...+ verb.