I heard "silence is a virtue" in Japan, so I pretended that I was incapable of speech while I was there.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means fake and the verb vocab version is similar, it means to lie or to cheat or to deceive.
Someone tries to lie to you. You can't believe it. It's what (いつわ)??? You can't believe it.