Green’s Dictionary of Slang

to attack physically.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 138/1: They got from six to nine months’ imprisonment; and those that let into the police, eighteen months.
1905		Sporting Times 4 Mar. 11/1: He lets into everything as hard as he can lick, and yet never makes a mis-hit or a mistake of any sort.