Green’s Dictionary of Slang

an intensifier.

1825		T.C. Croker Fairy Legends and Traditions of the South of Ireland (1862) 242: I was up, you see, murdering late last night.
1860		Poughkeepsie Jrnl (NY) 5 May 1/5: ‘You must fill me that room,’ says he, pointin’ into a murtherin’ big room.
1889		B. Mitford Fire Trumpet III 241: As it is it’ll be murdherin’ awkward.
1898		F.P. Dunne Mr Dooley in Peace and War 215: ‘Vell,’ says he (ye know th’ murdhrin’ way he has iv speakin’), ‘here we are,’ he says, ‘frinds at las’.’.
1917		E. O’Neill In the Zone (1923) 22: Look here, ye murrdherin’ swab.