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Injecting Structured Lexicographic Knowledge into LLMs for Non-Literal Expression Disambiguation: A Controlled Study on Croatian

In potentially idiomatic expressions (PIEs), the same surface form may receive either a literal or an idiomatic interpretation depending on context, making automatic literal–idiomatic disambiguation challenging. This is acute for Croatian, where annotated data and locally runnable generative models are limited. We present a study of Croatian PIE literal–idiomatic disambiguation examining how structured lexicographic knowledge can improve open-weight, decoder-only LLMs without fine-tuning. Using a new expert-annotated concordance dataset – CroPIEs, we compare baseline prompting to inference-time knowledge injection via retrieval-augmented generation (RAG) from a Croatian phraseological dictionary. We isolate the contribution of three knowledge types: definitional knowledge (structured meanings), contextual knowledge as curated prototypical usage examples, and their combination. Results show consistent improvements in macro-F1 for both GaMS-2B-Instruct and GaMS-9B-Instruct models. Definitional knowledge is generally more stable than examples alone, while examples can be effective but less consistent across expressions. The strongest and most reliable gains are obtained when definitions and examples are combined, indicating a synergistic effect between explicit meaning descriptions and contextual cues. Per-class analyses show that injected lexicographic evidence mitigates baseline biases between Literal and Idiomatic predictions, improving decision balance in a low-resource setting with small data of compact, expert-curated lexicographic evidence injected at inference time.