'penitential' originates from Late Latin, specifically the word 'paenitentialis', where the root 'paenitent-' (from 'paenitentia') meant 'repentance' and the suffix '-alis' meant 'relating to'.

'penitential' changed from Late Latin 'paenitentialis' into Medieval/Church Latin forms and entered Middle English (as 'penitentiall' or similar), eventually becoming the modern English word 'penitential'.

Initially it meant 'pertaining to repentance or penance', and over time it has largely retained this meaning, referring to acts, attitudes, or writings expressing remorse or penance.