onerous cause
Webster's Dictionary (GCIDE)
Onerous \On"er*ous\, a. [L. onerosus, fr. onus, oneris, a load, burden: cf. F. on['e]reux.] Burdensome; oppressive. "Too onerous a solicitude." --I. Taylor. [1913 Webster]
Onerous cause (Scots Law), a good and legal consideration; -- opposed to gratuitous. [1913 Webster]