What is the mean of “Quasi-steady Assumption”?

“Quasi-” is an Italian word meaning ‘about’. In English usage it usually has the sense of ‘almost’ or ‘sort of’. So you are saying “almost steady assumption”. I have no idea what that might mean. AFIK steadiness is not a property of assumptions.

To add to @SmartAZ, Quasi also has the French translation, ‘half’, hence the character Quasimodo in Victor Hugo’s ‘Hunchback of Notre Dame’ which means: ‘half-formed’.
No idea how any of its translations would apply to an assumption, however.