Lewis & Short

falsĭtas, ātis, f. [falsus], falsehood, falsity (post-class.), Lact. 5, 3, 23 (opp. verum); Amm. 15, 5, 12; Arn. 1, 33; Cael. Aur. Tard. 3, 4, 65; Isid. 1, 20, 3.
Plur., Arn. 1, 56; 7, 49.