On traditional accounts, word meanings are entries in a mental lexicon. Nonsense words lack such entries, and are therefore meaningless. Here, we show that under some circumstances nonsense words function indistinguishably from conventional words. The ‘nonsense’ words foove and crelch led participants to select systematically different clusters of adjectives and were reliably matched to different species of alien creatures (e.g., ‘crelches’ were pointy and narrow and ‘fooves’ were large and fat). In a categorization task in which participants learned to group two species of aliens primarily on the basis of roundness/pointiness, these novel labels facilitated performance as much as conventional words (e.g., round, pointy). The results expand the scope of research on sound symbolism and support a non-traditional view of word meaning according to which words do not have meanings by virtue of a conventionalized form−meaning pairing. Rather, the ‘meaning’ of a word is the effect that the word form has on the user’s mental activity.