This paper develops a dynamic theory of jargon as a mechanism of social closure. Insiders communicate to deepen productive knowledge, but their communication leaks knowledge to outsiders. Some outsiders become poseurs, able to mimic the first layer of insider language; some poseurs become amateurs, able to provide valuable service. We show that jargon is not simply a barrier to entry. It establishes a partially permeable boundary that regulates the speed and composition of knowledge diffusion. The model shows that a jargon equilibrium cannot involve perfect exclusion: if no outsider can ever decode the code, poseurs disappear, and jargon loses its strategic role. Nor can jargon be too easy: excessive leakage creates too many amateurs and undermines the scarcity rents needed to sustain communication. We highlight macro-environment factors that shape the equilibrium difficulty of jargon. Our analysis also reveals the social welfare role of jargon: when amateur knowledge is sufficiently productive, jargon can raise welfare by sustaining productive insider exchange while allowing controlled diffusion outside the formal insider path.