A-number-one, n. — Green’s Dictionary of Slang

A-number-one n.

also A No.1

[i.e. ‘the first’; var. on number one n. (1)]

(US) oneself.

1924		G. Henderson Keys to Crookdom 396: A No. 1. Used in place of the personal pronoun.
1936		J.T. Farrell World I Never Made 397: She just looks out for A-Number-One in every way that she can.