From Longman Dictionary of Contemporary Englishstart in phrasal verb American English1 to begin doing something, especially with a lot of effort I decided to just start in and see what I could do. on Lilly started in on her burger.2 to begin criticizing someone or complaining to them about something on Mom turned away from Rose and started in on me. → start→ See Verb table
Examples from the Corpus
start on• Then, realizing there was no point in trying to get up, she started in on another giggling jag.• Before I knew it, she'd started in on my wife.• The passageway outside was still reverberating from the crash when he started in on the door of the next apartment.start on• Then, realizing there was no point in trying to get up, she started in on another giggling jag.• The passageway outside was still reverberating from the crash when he started in on the door of the next apartment. 