Usually used in a restaurant setting, it bears the connotation of, "I'm behind you."

When someone has your back, they are there to support you unconditionally. When life seems to blindside you with undesirable events, they’re there for you without complaint, supporting you in your moment of need, not for their own selfish, self gratifying reasons, but because your well being to them is foremost in their mind and heart. A lover not only does the above, but is flexible in mind and heart to make the necessary changes and sacrifices in their own lives if they know it is something that enhances yours.
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Someone that has your back recognize the goodness, even the greatness in you, especially during the moments you’re not willing or able to see it. Simply stated, when someone has your back, your life is greatly enhanced.

This originally came from old movies and TV shows where two people are in a dangerous situation (usually hunting criminals or other enemies), and one of them is going to venture forward. The other person stays behind and watches for any danger coming from behind or other directions that the first person is unlikely to see. The one who stays behind typically has a weapon, to fend off any attackers.

When someone is really good at sex and leaves you sore the next day

1.Convincing a weak minded individual to handle a confrontation for you, usually through violence.
2. To persuade a individual to accomplish a task they were previously unaware of; or procrastinating on.

A new dance move that involves raising both of your hands in the air, and moving them side to side, to give the illusion that one is in fact drying their back with a towel. Variations in the word "dry" and "your" can indicate whether the act has happened, is happening, or is about to happen, and also who in fact is partaking in drying their back on the dance floor.

To have some very burdensome nuisance you have to put up with--in a sense wider than just a drug or other addiction.