A smoothie is a penis. Sometimes it protrudes through the pants and plays quite the eye candy, to say the least. It can be obvious, or sometimes an inconspicuous "smoothie". It's a codeword and no one will know what you're talking about :P. It's a rather nice word, which can correlate to a slurping sound one could make. Like drinking a smoothie through a straw. I don't know how my friend and I invented this word, but we use it, A LOT.
"Haha, Jake, Nick, and Alex all jerked it into a cup and made a smoothie!" - Nick
"Pics or it didn't happen." -Joe
"HAHA, We gave it to Jenny and we told her it was just her boyfriends jizz and she drank it!" - Nick
"OH GOD, Holy shit!" - Joe