A difficult obligation is a responsibility that is hard to fulfill. It can be a TASK or an ONUS that requires a lot of effort and dedication. The term 'difficult obligation' can be broken down into two words: 'ONUS' (4 letters) and 'TASK' (4 letters). An ONUS is a burden or responsibility that someone has to take on, while a TASK is an assigned job that needs to be completed. Together, these two words form the phrase 'difficult obligation, ' which describes a responsibility that is hard to fulfill.