not easily borne; wearing; "the burdensome task of preparing the income tax return"; "my duties weren't onerous; I only had to greet the guests"; "a taxing schedule"
mutability in life or nature (especially successive alternation from one condition to another)
a variation in circumstances or fortune at different times in your life or in the development of something; "the project was subject to the usual vicissitudes of exploratory research"
so extremely ugly as to be terrifying; "a hideous scar"; "a repulsive mask"
grossly offensive to decency or morality; causing horror; "subjected to outrageous cruelty"; "a hideous pattern of injustice"; "horrific conditions in the mining industry"
difficult to penetrate; incomprehensible to one of ordinary understanding or knowledge; "the professor's lectures were so abstruse that students tended to avoid them"; "a deep metaphysical theory"; "some recondite problem in historiography"