mori-
“sea, body of water”sea, lake, standing water
Root for sea/water body, yielding Latin mare, Old English mere (lake), German Meer.
Discussion
The PIE root *mer- (to die, to disappear, to waste away) produced the Indo-European vocabulary of death and mortality — a word family that has shaped legal, medical, philosophical, and religious discourse from the Vedas to modern insurance law.
Latin morī (to die) and its nominal derivative mors (death — genitive mortis) are the central Italic reflexes: mortal (subject to death), mortality (the condition of dying), immortal (not dying), mortify (to cause to die — later weakened to "to cause extreme embarrassment"), mortgage (literally morte-gage, "death-pledge" — a loan that "dies" when repaid or that seizes the property upon the borrower's death), and post-mortem (after death). The medical term moribund (about to die) and the legal term mortmain ("dead hand" — property held inalienably by a corporation) extend the family into professional vocabulary.
The compound *n̥-mr̥-tó- (not-dead, undying) gave Latin immortālis and, more strikingly, Greek ámbrotos (ἄμβροτος, "immortal") — from which ambrosía (ἀμβροσία, the food of the gods, "the undying stuff") derives. The negative prefix *n̥- (not) combined with the root for death to name divinity through its absence: the gods are defined by what they lack — death. English ambrosia preserves this PIE theology of immortality as a culinary term.
The Germanic branch shows the root in English murder (OE morþor, from PGmc *murþrą), with the specific sense of unlawful killing rather than death in general. German Mord (murder) and the related forms confirm the narrowed Germanic sense. The word mare (as in nightmare — OE mære, a crushing nocturnal spirit) may be distantly related, though this is debated.
Sanskrit mr̥tyú- (death) and the related márati (he dies) continue the root in Indo-Iranian. The god of death in Hindu mythology, Mṛtyu, bears the root as his name. The Avestan cognate confirms the Iranian reflex.
Greek brotós (βροτός, "mortal man" — from an earlier *mrotós with regular Greek dissimilation) preserves the root in a poetic register, used by Homer and the tragedians to distinguish humans (the dying ones) from gods (the undying). The opposition *mer-tó- (mortal) vs. *n̥-mr̥-tó- (immortal) may be one of the oldest conceptual pairs in PIE theology.
Old Irish marb (dead) and Lithuanian mìrti (to die) confirm the Celtic and Baltic reflexes. The root's universal distribution and consistent meaning make it one of the most secure PIE reconstructions — the word for death was, it seems, undying.
Notes
Pokorny 748. English marine, maritime, marsh.