early 15c., "deserving of divine grace," from Latin meritorius "that for which money is paid, that by which money is earned," from meritus, past participle of merere "to earn" (from PIE root *(s)mer- (2) "to get a share of something"). From late 15c. (Caxton) as "deserving of reward, worthy of praise or honor." Related: Meritoriously; meritoriousness.
Proto-Indo-European root meaning "to get a share of something."
It is the hypothetical source of/evidence for its existence is provided by: Greek meros "part, lot," moira "share, fate," moros "fate, destiny, doom;" Hittite mark "to divide" a sacrifice; Latin merere, meriri "to earn, deserve, acquire, gain."