'austerer' (as a comparative form) ultimately derives from Latin, specifically the word 'austerus', where the root meant 'harsh' or 'severe'.

'austerus' passed into Old French as 'austere' and then into Middle English as 'austere'; the modern English adjective 'austere' gave rise to comparative forms such as 'austerer' (by regular English comparative formation).

Initially it meant 'harsh' or 'severe' in a moral or disciplinary sense; over time it broadened to include 'plain, simple, or unadorned' as well as 'severe', and the comparative now conveys 'more severe' or 'more plain/simple'.