A patient understands everything said to him and clearly knows what he wants to say, but can only produce a handful of expletives and fixed phrases, delivered with different intonation each time to try to convey different meanings, visibly frustrated by his inability to say more. What is this?

This is expressive aphasia: difficulty producing spoken language despite generally intact understanding, with speech reduced in severe cases to a few expletives or fixed utterances delivered with varying intonation, exactly as described. It is not mutism, since expressive aphasia differs from mutism in that some speech output, however limited, is typically still present, including expletives or fixed utterances under emotional stress. It is not receptive aphasia, in which comprehension rather than production would be the chief difficulty, whereas here comprehension is clearly intact. And it is not thought blocking, which is a sudden, complete stop in an otherwise normal flow of thought and speech, rather than a persistent, severe difficulty producing any spoken language at all.