I’m speculating that “Last Out” refers to the newest and youngest child.
It is simple it means Little one 
Wow thanks so much dear 
I thought it was for a loved one, lol 
Random thought: Despite my understanding of what LO is, I don’t think many of these initialisms are required at all. I truly wish people would just write “kid” in this instance, just one more letter.
in answer to those who find the term appealing or who find the criticism of it unclear: use the term if you find it appealing! I mean, there’s enough tribalism in the parenting world; I have no right to disparage those who embrace the phrase. I detest the name itself.