The phrase "onerous task" means a difficult and burdensome job or assignment. It refers to a task that requires a lot of effort and can be very challenging. Full definition
* endured racial discrimination in their workplace where a two - tier system of treatment was in place with indigenous and Polish workers paid into their bank accounts and offered permanent employment, yet the Hungarians were given more onerous tasks, no security of employment and given cash handouts in brown paper envelopes. (politics.co.uk)
Ashcroft and the Tories have refused to answer questions about when Ashcroft fulfilled the less onerous task of declaring himself a long - term resident, which allowed him to continue to be a «non-dom» paying tax only on his UK earnings and avoiding giving tens of millions of pounds to the tax office on his substantial international estate. (theguardian.com)