1. To not put something away in its proper place. A noun or pronoun can be used between "leave" and "out." Don't leave your dirty clothes out in the middle of the living room! Aw man, I accidentally left out the ice cream—now it's all melted.

2. To withhold or omit something. A noun or pronoun can be used between "leave" and "out." Your paper is nicely written, but you left your references out. I did talk to Mom, I just left out the part about getting detention.

3. To ignore, forget, or exclude, as from a group, activity, benefit, etc. In this usage, the phrase is usually used in the past tense ("left out"). A noun or pronoun can be used between "leave" and "out." Of course I felt left out when you guys went to the concert without me! You know it hurts your brother's feelings when you leave him out of things.