When Does a Novel Form Become a Word? Causal Evidence for Reusable Lexical Function in Frozen Language Models

When does a novel form begin to function like a word? Inferring its meaning from context is not enough: lexical function emerges when the form itself becomes a cue for reusing semantic information inferred from its prior usages. We study this transition from contextual evidence to lexical function in frozen language models. Across Qwen3-8B and Gemma-3-4B, reversing the meaning assignments of two novel forms reverses which form the model selects for the same held-out situation. We then causally intervene on internal states produced by the preceding usages, separating their influence from the original examples. Transplanted states carry semantic influence from the source context into a new judgment, with a preferential effect on the form with which that meaning was associated. The same states also guide judgments of previously unseen usages, even when the source definition or concept label is no longer presented. This lexical function depends on how the relation was formed, not merely on which semantic material appeared in context. Holding source sentences, order, length, and form frequency fixed, more consistent form–meaning assignments produce stronger transferable semantic influence; subsequent target evidence then reduces the advantage established by stronger formation. Together, these results show that implicit usages do more than make a novel form interpretable: they organize usage-derived semantic information into a relation that can be reused through the form itself. We call this history-dependent, reusable, and evidence-sensitive form–meaning relation a revisable lexical hypothesis.