Green’s Dictionary of Slang

aggro n.

also agro
[abbr. SE aggravation + -o sfx (6)]
[1960s+]

1. problems, trouble.

2. violence, typically as enjoyed by skinheads, esp. at football matches, beating up Asians etc.

3. an aggressive attitude.

4. any form of problems, difficulties, harassment.

In phrases

go aggro (v.)

[1980s+] (US campus) to become aggressive.