It is the combination of the words 'bro' and 'mermaid'.
Woman - "So you're a guy and you've seen 'The Little Mermaid'?"
Man - "I sure have."
Woman - "So, you have a daughter then."
Man - "Nope. I just love the movie AND I have lots of Ariel swag, too."
Woman - "So, you're gay then."
Man - "Nope. I'm just a big ol' Bromaid!"