Green’s Dictionary of Slang

1. an affectionate term of address.

1942		(con. 1890s) S. O’Casey Pictures in the Hallway 192: Now for the theatre, me laddo, to run over the words, and give you a feel of the stage.
1962		H. Livings Nil Carborundum (1963) Act III: Now then my laddo, we’ll have a bit of a check about you.

2. a hooligan.

1998		K. Sampson Awaydays 33: The bad boys still turn them on, but it isn’t cool to act the laddo in a place like that.
2000		J. Hawes Dead Long Enough 22: If you meet one of these laddos or saddos coming round the right-hand side of a bend.