Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
masturbor, 1, v. dep. [perhaps from manus-stupro], to defile one’s self (postAug.), Mart. 11, 104, 13; 9, 42, 7; cf. in the act. form masturbat, δέφει, δέφεται, Vet. Gloss.