horn in on

Intrude, join without being invited. For example, She has a rude way of horning in on our conversations. This expression alludes to an ox or bull pushing in with its horns. [c. 1900]

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