start in on

1

Begin doing something, as in We started right in on the repairs. Also see start in.

2

Attack, especially verbally, as in Nancy keeps starting in on Carl, complaining about the errors in his work. [Early 1900s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.