1. To rest, press, or prop against someone or something while bending or reclining backwards. She leaned back against the tree and closed her eyes for a few minutes. I started leaning back against the man to push him out of the doorway.

2. To rest, press, or prop someone or something against someone or something while bending or reclining backwards. In this usage, a noun or pronoun is used between "lean" and "back." I leaned the injured man back against the wall so he could tell me what happened. Just lean the mirror back against the sofa while we're painting the wall.