A medical prescription, of which the Rx symbol is customarily part of the superscription, is a legal order by a qualified health care professional to a pharmacist or other therapist for a treatment to be provided to their patient.
There are several theories about the origin of the Rx symbol. Most agree, Rx is an abbreviation for the Latin word recipere, which means "take" or "take thus." Long ago, this would not have been a direction to a patient but to a pharmacist, preceding the physician's "recipe" for preparing a medication. In the days before manufactured drugs, apothecaries (who were also doctors) would write out a formula for medications. They would mix up and compound ingredients to make drugs or remedies. Not until well into the 19th century was the distinction between the apothecary/pharmacist as a compounder of medicines and the physician as a therapist generally accepted.
Theory number two, from the book Devils, Drugs, and Doctors, suggests "Rx is not, as is frequently supposed, an abbreviation of a Latin word meaning recipe or compound, but is an invocation to Jupiter, a prayer for his aid to make the treatment effective... sometimes in old medical manuscripts all the R's occurring in the text were crossed." So basically, the Rx symbol was a corruption of the ancient symbol for the Roman god Jupiter, which has a very similar crossed leg at the bottom right.
And then there are those who say the pharmaceutical symbol used to be an EYE with an "x" below it instead of the "R," and was called the "Eye of Horus." According to this school of thought, the Egyptian god Horus was the "father of pharmacy." Slide over Jupiter! Horus was an Egyptian god, the god of Nekhen, a village in Egypt, and god of the sky, of light, and of goodness. He was the son of Isis, the nature goddess, and Osiris, the god of the underworld. Osiris was murdered by his evil brother Seth, the god of darkness and evil. Horus sought to avenge his father's death by challenging his uncle Seth to a fight. Seth cut out Horus's eye, but Thoth, a god associated with wisdom and compassion, magically restored the eye. Horus did defeat Seth, finally. Horus's eye, also called the wadjet eye, became a pwerful symbol for health.
Celebrate the various theories on the origin of the Rx symbol, honor the compounding abilities of pharmacists, or more importantly acknowledge the fact that all the good shit requires a prescription with an Rx Symbol T-Shirt!