to agree with or accept as valid (an argument, theory, etc)
Australian and NZ informal to get involved in (an argument, fight, etc)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Other Idioms and Phrases with buy into
Purchase a membership, a share, or an interest in something. For example, I'd love to buy into this partnership, but I can't afford it. [First half of 1600s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.