There is a subtext to certain conversations, especially between knowing friends that hints at exactly what neither of you is willing to say outright.
This is at it's very best when conducting a conversation with a third party or parties who are not as sharp or as telepathic as the aforementioned friends. This usually results in either successful containment of laughter and odd looks from the third party, or outright breaking down into hysterics, at which point you must rapidly pull yourself back together so as not to explain yourself.
There are classic implementation of innuendo, such as ...or are you just happy to see me?, or that's ok, I'll let you pay me back for it later. But the truly classic incidents are truly unique and are the subject of similar conversations for years to come.