jack: hey ted, why don't you invite your brother-in-theory onto our softball team?  ted: are you kidding? then my father-in-theory's gonna want to join too
a would-be brother-in-law who is in a commited relationship with your sister but isn't legally married to her
"jack: hey ted, why don't you invite your brother-in-theory onto our softball team?  ted: are you kidding? then my father-in-theory's gonna want to join too" What is the definition of "brother-in-theory"?