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gallon (n.): English measure of capacity (containing four quarts), usually for liquids, late 13c., from Old North French galon, corresponding to Old French jalon, name of a liquid measure roughly equivalent to a modern gallon," which is related to (perhaps augmentative of) jale "bowl," from Medieval Latin or Vulgar Latin diminutive form galleta "bucket, pail," also "a measure of wine," a word of unknown origin, perhaps from Gaulish galla "vessel."
magnum (n.): 1788, "bottle containing two quarts of wine or spirits," from Latin magnum, neuter of magnus "great in size" (see magnate). Registered 1935 by Smith & Wesson Inc., of Springfield, Massachusetts, as the name of a powerful type of handgun.
P: a rare letter in the initial position in Germanic, in part because by Grimm's Law PIE p- became Germanic f-; even with the early Latin borrowings in Old English, -p- takes up a little over 4 pages in J.R. Clark Hall's "Concise Anglo-Saxon Dictionary," compared to 31 pages for B and more than 36 for F. But it now is the third-most-common initial letter in the English vocabulary, and with C and S comprises nearly a third of the dictionary, a testimony to the flood of words that have entered the language since 1066 from Latin, Greek, and French.

To mind one's Ps and Qs (1779), possibly is from confusion of these letters among children learning to write. Another theory traces it to old-time tavern-keepers tracking their patrons' bar tabs in pints and quarts. But see also to be P and Q (1610s), "to be excellent," a slang phrase said to derive from prime quality.