See fall behind.
Support someone or something; also, help to promote someone or something. For example, We must find as many workers as we can to get behind the union.
Enjoy, appreciate, as in Norma just can't get behind ballet. [Slang; c. 1970]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.