'Abo' is an old Australian colloquial abbreviation of 'Aboriginal' that is now widely considered a racial slur against Aboriginal Australians and is offensive in current usage. If your child uses it, this is a moment for a calm conversation about why it is hurtful: it dehumanises Aboriginal people and is associated with a long history of racism in Australia. The correct, respectful terms are 'Aboriginal', 'Aboriginal Australian', 'First Nations' or 'Indigenous'. Many people who use 'abo' have heard it from older relatives without realising the harm; education tends to be more effective than punishment.