A Hard Nut to Crack: Idiom Detection with Conversational Large Language Models
Summary
The paper asks whether small conversational LLMs can tell literal uses of Potentially Idiomatic Expressions (PIEs) from figurative ones in English sentences. Existing benchmarks such as MAGPIE may have leaked into pre-training data, so the authors built a new test suite from scratch, IdioTS. They compiled 93 colloquial idioms covering a range of syntactic patterns. Native-speaker language professionals then wrote new idiomatic sentences for them. The researchers themselves added "distractor" sentences, in which idiom words appear with a literal sense.
Idiom detection is framed as binary sentence classification. Three instruction-tuned, open-source models of roughly 7B parameters were prompted to decide whether a sentence contains an idiom and to explain it. The outputs were evaluated both automatically and manually, followed by an error analysis.
All models struggled with ambiguous sentences. They showed a strong acquiescence bias: they "found" idioms where none existed and gave fluent but wrong justifications. This matters because it shows that apparent language understanding in chatbots can mask unreliable handling of figurative meaning.
Key points
- IdioTS composition: 250 English sentences, of which 164 are idiomatic (positive class) and 86 are distractors (negative class). It is released under CC BY-SA-NC 4.0.
- Sentence authoring: idiomatic sentences came from eight volunteer native English speakers at C1+ level, mostly British, including teachers, linguists, translators and NLP experts. The text was also gender-neutralised where possible.
- Distractor strategies:
- placing the full idiom in a context that forces a literal reading;
- reusing only some of its words;
- shifting a word's syntactic or morphological role;
- using sound-alike phrases, e.g. "speed and span" vs. "spick and span".
- Models: Llama2, Mistral and Vicuna, all ~7B-parameter dialogue models, run on a local machine without a GPU.
- Main error type: distractors where the model wrongly gave the associated PIE an idiomatic reading. Counts were 55 for Llama2 (0.640), 53 for Mistral (0.616) and 47 for Vicuna (0.546). Vicuna made the fewest such errors.
Cited by
- ID10M-JAM: Stress-Testing Idiom Identification Under Challenging Context
- ITUNLP at MWE-2026 AdMIRe 2: A Zero-Shot LLM Pipeline for Multimodal Idiom Understanding and Ranking
Cite
@inproceedings{fornaciari2024hard,
title = {{A Hard Nut to Crack: Idiom Detection with Conversational Large Language Models}},
author = {Fornaciari, Francesca De Luca and Altuna, Begoña and Gonzalez-Dios, Itziar and Melero, Maite},
booktitle = {Proceedings of the 4th Workshop on Figurative Language Processing (FigLang 2024)},
year = {2024},
doi = {10.18653/v1/2024.figlang-1.5},
url = {https://doi.org/10.18653/v1/2024.figlang-1.5}
}References
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