The only free and comprehensive online etymological dictionary of the Spanish language
merced f. (Noun) "mercy;" "grace"
12th cent. borrowing from Medieval Latin mercedem, accusative of merces "pay," "reward." From Latin merx "merchandise." From Proto-Italic *merk- "trade." From an unknown Etruscan word.
Indo-European
Romance
Portuguese mercê, French merci, Italian mercede
Italic
Latin Mercurius "god of trade," Oscan mirikui (name of a deity), Faliscan mercui (name of a deity)