Green’s Dictionary of Slang

(US) an imper. calling on one’s attention, esp. before delivering some reprimand or lecture.

1911		G. Bronson-Howard Enemy to Society 16: ‘And looka here!’ The paragraph he had annotated was in the accounts of real-estate transfers.
1969		H. Rap Brown Die Nigger Die! 119: Look-a-here. I’ll give you three.