Green’s Dictionary of Slang

(US) to stab.

1824		J. Wight Mornings in Bow St. 253: [H]e felt about the round table [...] in the hope of finding a knife. ‘And what would you have done with it if you had found it?’ asked his worship. [...] ‘I’d have dagged it into the heart of ’em at that same time!’.
1978		(con. WWII) T. Sanchez Hollywoodland (1981) 70: The Zolot waved the blade [...] ‘Back off or I’ll dag you!’.