The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
mer-
DEFINITION:
To rub away, harm. Derivatives include nightmare, morsel, morbid, mortal, mortgage, and ambrosia. I.1.nightmare, from Old English mare,mære, goblin, incubus, from Germanic *marn-, goblin. 2.marasmus; amaranth, from Greek marainein, to waste away, wither. 3. Probably suffixed zero-grade form *m-to-, “ground down.” mortar, from Latin mortrium, mortar. 4. Possibly extended root *merd-.mordacious, mordant, mordent, morsel; premorse, remorse, from Latin mordre, to bite. 5. Possibly suffixed form *mor-bho-.morbid, from Latin morbus, disease (but this is more likely of unknown origin). II. Possibly the same root is *mer-, “to die,” with derivatives referring to death and to human beings as subject to death. 1. Zero-grade form *m-.a. Suffixed form *m-tro-.murder, from Old English morthor, murder, from Germanic suffixed form *mur-thra-;b. suffixed form *m-ti-.mort1, mortal; amortize, mortify, postmortem, from Latin mors (stem mort-), death; c. suffixed form *m-yo-.moribund, mortgage, mortmain, mortuary, murrain, from Latin mor, to die, with irregular past participle mortuus (< *m-two-), replacing older *m-to- (for which see d); d. prefixed and suffixed form *-m-to-, “undying, immortal.” (*-, negative prefix; see ne). (i)immortal, from Latin immortlis;(ii)ambrosia, from Greek ambrotos, immortal, divine (a- + -mbrotos,brotos, mortal); (iii)amrita, from Sanskrit amtam, immortality (a- + mta-, dead). 2. Suffixed o-grade form *mor-t-yo-.manticore, from Greek mantikhras (corrupted from marti(o)khras), manticore, probably from Iranian compound *martiya-khvra-, “man-eater” (*khvra-, eating; see swel-), from Old Persian martiya-, a mortal man. (Pokorny 4. mer-, 5. mer- 735.)
n-, goblin. 2. marasmus; amaranth, from Greek marainein, to waste away, wither. 3. Probably suffixed zero-grade form *m
-to-, “ground down.” mortar, from Latin mort
rium, mortar. 4. Possibly extended root *merd-. mordacious, mordant, mordent, morsel; premorse, remorse, from Latin mord
re, to bite. 5. Possibly suffixed form *mor-bho-. morbid, from Latin morbus, disease (but this is more likely of unknown origin). 
, to die, with irregular past participle mortuus (< *m
-m
tam, immortality (a- + m
