Onerous, a.
Etymology: [L. onerosus, fr. onus, oneris, a load, burden: cf.F. onéreux.]
Burdensome; oppressive. "Too onerous a solicitude." I. Taylor. Onerous cause (Scots Law), a good and legal consideration; -- opposed to gratuitous.
Shades of meaning
Fernald has no entry of its own for onerous; he lists it under difficult.
Where he writes about it:
That which is onerous (L. onus, a burden) is mentally burdensome or oppressive. — from difficult
Responsibility may be onerous even when it involves no special exertion. — from difficult