In the Devil's Dictionary
by Ambrose Bierce we find the following entry:
R.I.P. A careless abbreviation of requiescat in pace, attesting an indolent goodwill to the dead. According to the learned Dr. Drigge, however, the letters originally meant nothing more than reductus in pulvis.
The fictitious Dr. Drigge was not learned in Latin grammar. This should be "reductus in pulverem," not "reductus in pulvis." The phrase means "reduced to dust, or ashes." Pulvis is nominative and cannot be the object of a preposition. All of the oblique cases show the stem "pulver," whence English "pulverize."