If a model doesn't "know" what a PM is then I worry about any of its other outputs. That should be dictionary lookup.

Why? It's honest as it doesn't understand it without more context. Lookup could lead to wrong results

Seriously. I have never seen this, even once. I had been wondering if it was impossible. If a model can really say “I don’t know” when it doesn’t know, that could change everything. How many pointless, dumb rabbit holes could be avoided?