opus facere (De Senect. 7. 24)

Etymology:

• opus always means the concrete work on which one is engaged; labor is the trouble, fatigue, resulting from effort; opera is the voluntary effort, the trouble spent on an object. Thus laborare = not simply to work, but to work energetically, with exertion and consequent fatigue; operari, to be busy with a thing. Terence thus distinguishes opus and opera: quod in opere faciundo operae consumis tuae. Cf. Verg. Aen. 1. 455 operumque laborem miratur = the trouble with such huge works must have cost.