What, do you suppose, was once called in French pissenlit, in English piss-a-bed? It was nothing of a sexual nature. The piss-a-bed was, in fact, the simple dandelion. An innocent flower, the piss-a-bed thrives everywhere. The dandelion was not so called for its color, or its tendency to blow in the wind. No: the dandelion is a diuretic.
The connection between the words leak and piss is interesting. Henry Miller’s Tropic of Cancer seems to contain the first use of leak as an act of urination, i.e. a thing that one may take. However, leak has been used as a verb meaning to piss since the 1590s. In Act II of Shakespeare’s Henry IV (Part I), a carrier exclaims: “Why, you will allow vs ne’re a Iourden [chamber-pot], and then we leake in your Chimney.”
To piss in somebody’s chimney is impolite, but not so impolite perhaps as to reveal sensitive information. That meaning of leak is young—since the ‘50s only—and appears to derive from simple analogy to the ordinary meaning, of stuff escaping through a hole. By contrast, the verb to piss is as old as the 13th century; as a noun, piss is a slightly younger sister, dating only to the 14th.
To piss derives ultimately from the Vulgar Latin verb pissiare. The proper Latin verb meaning to urinate was mingere, which gives us medical words like micturition. Via the medieval French verb pissier (12th century), to piss crops up in many medieval English texts. For example, in Chaucer’s “Wife of Bath’s Tale,” he writes of Xanthippe pouring a chamberpot upon the head of her husband Socrates: “Xantippa caste pisse vpon his heed.” After this episode Socrates is supposed to have mused, “After the thunder, comes the rain.”
Despite sharing a majority of letters, the words “piss” and “pussy” are not etymological relations. Pussy (n) may derive from the Old Norse puss meaning pocket, or pouch.