go at it

Short definition from Wiktionary

Verb

1. 1
   As go at + it; spoken emphasis typically on 'go'.
2. 2
   As go + at it; spoken emphasis typically on 'at': To engage in some activity, especially vigorously or enthusiastically.
3. 3
   As go + at it; spoken emphasis typically on 'at': To fight.
4. 4
   As go + at it; spoken emphasis typically on 'at': To have sex.