The general wisdom is that tip stands for To insure promptness. The general wisdom is especially unwise in this case, considering insure is what insurance companies do - the word that means to make certain of is ensure. The actual etymology is much more mundane - it's probably descended, in a roundabout way, from the Low German tippen, meaning to tap, according to www.etymonline.com.
To me, tipping adequately is an ethical obligation, and the hosts of the podcast Awesome Etiquette podcast have perfectly articulated my case. Co-host Daniel Post Senning points out that when one eats at a restaurant, there is a tacit contract between the server and the patron - an understanding that the patron will pay the server for their services. So stiffing someone, failing to leave a tip, is a violation of that contract. Using stiff to mean fail to tip first shows up in print in 1939 - possibly related to stiff meaning jerk or bad person, but I can't figure out exactly how stiff came to mean jerk. It came to refer to a dead person in 1859, for obvious reasons, but dead people aren't really known for being jerks, what with being dead. Though dead people are known for being lousy tippers, just ask any mortuary assistant.
By the way, when people tell me that they can't afford to tip, I lose pretty much all respect for them. It's like shoplifting a necklace and claiming it's okay because you couldn't afford it. If you can't afford to pay for something you don't need, then you can't have it, that's life. Your server certainly can't afford to do their job on their salary alone.
By the by, the Awesome Etiquette people say that if you have terrible service, the most polite way to handle it is to leave a minimum 15% tip, but then complain to management.
The folks at www.businessinsider.com offer this handy Infographic on tipping: