Green’s Dictionary of Slang

ABC n.¹

[? play on ABC bread company, i.e. one’s legs have been toasted]

(Irish) scorch marks on one’s legs.

1927

P. O’Donnell Islanders (1933) 154: It’s the ashes bes on yer toes, an’ the ABC on yer shins with toastin’ them in the fire.