A disagreeable obligation is an unpleasant task or responsibility that one must fulfill. The term 'ONUS' is a four-letter word that can be used to describe this type of obligation. It is derived from the Latin word 'ONUS, ' which means 'burden.' ONUS is often used in legal contexts to refer to the burden of proof or responsibility. In everyday language, it is used to refer to any unpleasant task or responsibility that one must fulfill.