"that works, working, engaged in action," early 15c., from Latin operantem (nominative operans), present participle of operari "to work, labor" (in Late Latin "to have effect, be active, cause"), from opera "work, effort," related to opus (genitive operis) "a work" (from PIE root *op- "to work, produce in abundance"). Psychological sense of "involving behavior modification" coined 1937 by U.S. psychologist B.F. Skinner (as in operant conditioning, 1938, Skinner).
1540s, "one who enters into a contract," from Late Latin contractor "one who makes a contract," agent noun from past-participle stem of Latin contrahere "to draw several objects together; draw in, shorten, lessen, abridge," metaphorically "make a bargain, make an agreement," from assimilated form of com "with, together" (see con-) + trahere "to draw" (see tract (n.1)).
From 1680s as "a muscle which contracts a part." Specifically "one who enters into a contract to provide work, services, or goods at a certain price or rate" is from 1724.
It forms all or part of: allergic; allergy; argon; boulevard; bulwark; cholinergic; demiurge; dramaturge; energy; erg (n.1) "unit of energy;" ergative; ergonomics; ergophobia; George; georgic; handiwork; irk; lethargic; lethargy; liturgy; metallurgy; organ; organelle; organic; organism; organize; orgy; surgeon; surgery; synergism; synergy; thaumaturge; work; wright; wrought; zymurgy.
It is the hypothetical source of/evidence for its existence is provided by: Greek ergon "work," orgia "religious performances;" Armenian gorc "work;" Avestan vareza "work, activity;" Gothic waurkjan, Old English wyrcan "to work," Old English weorc "deed, action, something done;" Old Norse yrka "work, take effect."