Green’s Dictionary of Slang

1. someone important, or posing as such; in phr. I’m the boy, I am the right person for the job.

1836		T. Haliburton Clockmaker (1843) I 178: He’ll pay up the great folks this hitch, he’ll let ’em have their own, he’s jist the boy that can do it.
1857		Dickens Little Dorrit (1967) 100: ‘Childbed?’ said the doctor. ‘I’m the boy!’.
1959		W. Hall Long and the Short and the Tall Act II: And so you pull the big brave hero bull [...] Aren’t you the boy?
2004		Wiley ‘Next Level’ 🎵 Wiley makes it happen. I happen to be the boy round here.

2. the accused.

1956

H. Ellison ‘Johnny Slice’s Stoolie’ in Deadly Streets (1983) 81: If it didn’t, they figured I was the boy.