Green’s Dictionary of Slang

a professional mercenary soldier.

1967		Time 11 Aug. 28/2: Zambesi Club ‘mercs’ are white Rhodesians and South Africans from Colonel ‘Mad Mike’ Hoare’s Fifth Commando.
1979		High Times Jan. 108: A squad of Dutch and West German mercenaries invaded the platform and took possession — on behalf of the foreign minister himself. [...] Six days later, Bates and four other armed men retook Sealand and ousted the mercs.
1999		W. Gibson All Tomorrow’s Parties 199: He had a feeling the scarf was the one he’d really have to watch for; he couldn’t say why. ‘What if those mercs scope us leaving?’.
2019		S.M. Jones Lives Laid Away [ebook] ‘I’m pretty sure you and your merry band of mercs had everything to do with the disappearance of Barney Olsen, Esquire’.