sincero
Could not find sinceres in Logeion dictionaries.
sinceres present active subjunctive 2nd singular of sincero
sincēro, āvi, ātum, 1, v. a. [sincerus],
- to make clean or pure, Diom. 361 P.
See: sincerare
sincērō, āre (sincerus), tr., rendre pur : Diom. 364, 28.