Douglas Harper's Etymology Dictionary
champertous
1640s, from champart, from French champart "portion of produce received by a feudal lord from land held in lease from him" (13c.), from Old North French campart-, probably from Latin campi pars "part of the field" (see campus + part (n.)). In later use often with reference to champerty (early 14c.), the illegal act whereby a person makes a bargain to maintain a litigant in return for a share of the gains if the case succeeds.
Wiktionary
champertous
a. Having the quality of champerty, being an instance of champerty.