(Acronym) Before Most Others.
Your BMO is someone you cherish over your best friends and even your besties. They are at the top of your social list, just underneath your SO (if you have one.)
“Hold on Bestie, I’m getting a call from my boyfriend”
“Yeah, I told her it was my boyfriend because I didn’t want to make her feel bad because she wasn’t my BMO.”
Usually used on the internet to tell somebody that they want to smoke without potential teachers, parents, family members knowing. Can also be interpreted in the same way as "Smoke me out".