be on about sth.是惯用短语，如果一定要分析的话，on是副词作表语，about构成介词短语作方面状语。你查一下词典就知道了。比如《牛津高阶英汉双解词典》

be/go/keep on about sth (infml derog 口, 贬) talk in a boring, tedious or complaining way about sth唠叨; 抱怨: What's he on about now? 他在絮叨什麽? 

又如：

Honest, Kate, I don't know what you're on about. 说实话，凯特，我不知道你在说什么。《麦克米兰英语词典》