Loose is a broad term used to define a variety of acts. the words daring, bold, stupid, hilarious and retarded can be used to define the term. It catches on.
"Damn, Mark just had sex with my girlfriend"
"wow, thats loose"
"Wow, Jane just drag raced a cop"
"word. shes loose as hell"
"Last night Bobby drank 90 beers"
"hes a loose dude"