What is the meaning of 吃瘪? I'm sure it's not good, but what does 吃瘪 mean?

德国人的克星其实是Siri，只有Siri能让德国人吃瘪。

Germans' bane is Siri, only Siri can make Germans 吃瘪。
Imagine something sharp cut through a basketball, as a result, the air in that ball was gone and the ball became “瘪”. Also “瘪” can be used to described a person, e.g. “他营养不好，身体干瘪” means "He looks not that healthy and excessively slim".

When saying someone “吃瘪”, it is similar to the usage above. It means someone was frustrated probably because he was treated unfairly and couldn't even defended himself. That person looks like the basketball as we said above. In Chinese, you can not only “吃” food, but also “吃” something else, e.g. “吃亏”、“吃豆腐”

Another Chinese idiom I think is equivalent to “吃瘪” is “哑巴吃黄连，有苦说不出” (Hint: “黄连” is a kind of herb which is bitter).

By the way, here “瘪” is pronounced as "biē" not "biĕ".

EDIT:

I don't know what the context of your sentence is. I guess the reason why Siri make Germans “吃瘪” is her awkward or ridiculous AI response? If I'm correct, I will translate the sentence to "only Siri can make Germans hesitate or be unable to make a response". "Well... seriously?" To my knowledge, I can't find an English word to replace “吃瘪”.