How are the forms of our communicative behaviors related to their meanings? Iconic form-meaning relationships are commonly found in people's co-speech gestures: Many gestures resemble the objects or events they refer to. Iconicity has also been observed to a lesser extent in signed languages, and to an even lesser extent in spoken languages, for which form-meaning relationships are assumed to be largely arbitrary. The assumption of arbitrariness has persisted, in part, because the search for iconic form-meaning relationships has focused primarily on signs with concrete (visible, tangible) referents: literal iconicity. Yet, via metaphor, it is also possible to create iconic form-meaning relationships between signs with abstract (invisible, intangible) referents: metaphoric iconicity. The studies reported here demonstrate previously unrecognized iconicity in gesture, sign, and spoken language. Study 1 shows iconic form-meaning relationships in a type of gesture long thought to be devoid of meaning, beat gestures. Blind, quantitative analyses of more than 5000 spontaneous gestures showed that when people told stories implying motion or extension in space their beat gestures reflected the directions implied by the accompanying speech, no matter whether space was used literally or metaphorically. Study 2 reveals metaphoric iconicity spread throughout the lexicons of three signed languages (American Sign Language, French Sign Language, and British Sign Language), and Study 3 shows a similar pattern of metaphoric iconicity in a spoken language, Mandarin Chinese. Together, these studies suggest that the extent of iconicity in human communicative signals has been underestimated. The hidden iconicity found in beat gestures reveals the automaticity with which people activate mental representations of physical space to scaffold even highly abstract thoughts. The hidden iconicity found in words shows that spatial metaphors constrain form-meaning relationships across the lexicons of signed and spoken languages.