"Taxi" and "tax" are related, although it's necessary to look back through many years and several languages to see the exact point at which they connect, say editors at Merriam Webster Inc. The word "taxi," short for "taxicab," can actually be traced to the name of the device used to keep track of those costly fares. Most people now just call that device the "meter" (as in "keep the meter running"), but its full name is "taximeter," a word derived from the French "taximetre," the German "taxameter," and ultimately from the Latin "taxa," which means "tax."
The taximeter was invented in Germany in the late 19th century and was soon afterward adopted in France. When vehicles equipped with taximeters first appeared on the streets of London in the early 20th century, they were variously called "taximetercabs," "taximos," "taxi-cabs," and "taxis." No doubt people complained about the high fares then, too.
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Talk about oats
"Feel one's oats" is an old farmer's expression, recorded in print as early as 1831, when it appeared in a Boston newspaper, say editors of Merriam Webster, Inc. The phrase originally and literally described horses made lively and energetic after feeding on oats instead of wheat or grass. Within only a few years after its first use in print, the phrase started appearing in an extended sense applied to energetic people. It now seems to be used almost exclusively to refer to people.
A less common expression if "full of oats," which has much the same meaning. We've also seen "feel one's beans" and "full of beans," similarly derived. Slightly different is "smelling one's oats," meaning moving with new energy once one's goal is within sight.