Abstract: In view of the popularity of Second Language Acquisition, this study examines a related area of inquiry, the incidental English lexical acquisition. It aims at investigating the potential differences in employing different linguistic knowledge sources by adult male Jordanian learners of English, when acquiring English lexis incidentally. To this end, a lexical inferring test was used to instigate previously knowledge in morphology, phonology, syntax and semantics acquired by 16 Jordanians. Data were then analyzed quantitatively and categorized according to the employed linguistic knowledge sources. Results showed that participants tended to rely more heavily and successfully on their previous knowledge in semantics followed by morphological analysis, whereas reliance on syntax was least used, followed by dependence on phonological relationships which was the most misleading. Differences appeared in terms of the frequency of using each linguistic knowledge source as well as the effectiveness of making use of each source. Differences can be ascribed to variance in length of individual's previous English learning experience. The study highlights the importance of knowledge in linguistics as a prerequisite to facilitate lexical acquisition. It also has implications for lexical studies and second language acquisition.