ONlwSG

²bàgh m. [b̥aːɣ], gen. bàigh [b̥aːj]; cf. EG báġ f. ‘boast, threat, promise, undertaking, declaration; fight, contest, striving, act of contending etc.; warlike qualities (?), boldness’ (eDIL^˄). SG bàgh ‘promise, bond, tie; strength, power, virtue; word; battle’ is listed by Dwelly (1911) as obsolete, 

Shaw (1780) lists baghadh ‘fighting, quarrelling’, but this is listed as obsolete by Armstrong (1825); HSS (1828) lists bagh [sic] ‘battle’, citing Lhuyd and Shaw: Shaw includes ‘bagh’ under fight, while Lhuyd 1707 lists †bágh (‘cath, battle’) in his Irish-English Dictionary but †bagh [sic] with similar senses in his Comparative Glossary (pp. 126, 131).

but MacLennan (1925) records bàgh ‘purpose’ and a dh’aon bhàgh ‘of set purpose’, while AFB^˄ records the word in the senses ‘power, virtue; promise, bond’ from Lewis and Skye.

Craigie (1894, 160) derives OG báġ and Cameron (in MacBain 1894a, 617) SG bagh (sic) from ON bágr (leg. bág acc.) m. ‘resistance, struggle’ (NO), and, although MacBain does not include the etymology in his own dictionary (1896), McDonald (2009, 340) considers the loan likely. However, Marstrander (1915a, 121) points out that the Gaelic word is demonstrably older than the Viking period as it occurs in the mid-8th-century Würzburg glosses: coṁraṁa et baġa Poil (Gluaiseanna Gaeilge Würzburg^˄, folio 30d, line 12) ‘Paul’s conflicts and battles’.