This article reports on the role pragmatic inferencing plays in accounting for the ways in which native speakers perceive and interpret the semantic transparency of idioms. Although previous studies have suggested that semantic transparency intuitions of idioms are partly motivated by the conceptual metaphors that underlie them (Gibbs 1992; Gibbs et al.1997), findings from other studies (Keysar & Bly 1995, 1999) have raised questions concerning the arbitrariness of such intuitions. This study seeks to further address the discussion on the nature of semantic transparency by examining the role of pragmatic inferencing and encyclopedic world knowledge for understanding how native speakers interpret the relationship between the literal parts and figurative meanings of metaphorical idioms. To this end, semantic transparency ratings were elicited among fifteen native speakers of English for 222 metaphorical English idioms. Furthermore, raters provided qualitative support by justifying their ratings for a smaller subset of 30 idioms. These initial results were then triangulated by a follow-up exploratory study surveying etymological notes from a number of idiom dictionaries. The findings suggest that pragmatic inferencing via encyclopedic world knowledge plays an important role for the non-arbitrary way in which native speakers perceive the semantic transparency of idioms.