Parabellum (n.)

early 15c., "inclined to fighting," from Latin bellicosus "warlike, valorous, given to fighting," from bellicus "of war," from bellum "war" (Old Latin duellum, dvellum), which is of uncertain origin.

The best etymology for duellum so far has been proposed by Pinault 1987, who posits a dim. *duelno- to bonus. If *duelno- meant 'quite good, quite brave', its use in the context of war ( bella acta, bella gesta) could be understood as a euphemism, ultimately yielding a meaning 'action of valour, war' for the noun bellum. [de Vaan]

*perə-, Proto-Indo-European root meaning "to produce, procure" and yielding and derived words in diverse senses; possibly related to *pere- (2) "to grant, allot."

It forms all or part of: ante-partum; apparatus; apparel; biparous; disparate; emperor; empire; heifer; imperative; imperator; imperial; juniper; multiparous; nulliparous; oviparous; para- (2) "defense, protection against; that which protects from;" Parabellum; parachute; parade; parados; parapet; parasol; pare; parent; -parous; parry; parturient; poor; post-partum; preparation; prepare; primipara; puerperal; rampart; repair (v.1) "to mend, put back in order;" repertory; separate; sever; several; spar (v.); viper; vituperation; viviparous.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prthukah "child, calf, young of an animal;" Greek poris "calf, bull;" Latin parare "make ready, prepare," parire "produce, bring forth, give birth to;" Czech spratek "brat, urchin, premature calf;" Lithuanian periu, perėti "to brood;" Old High German farro, German Farre "bullock," Old English fearr "bull."