伪(wěi) - false, fake, counterfeit, bogus in Mandarin

伪 (wěi) means false or counterfeit, in the sense of something deliberately faked. It is a bound morpheme in modern Mandarin: you almost never meet it alone, but it is everywhere inside compounds. 伪造 is to forge, 伪装 is to disguise, 虚伪 is hypocritical, 伪钞 is a counterfeit banknote, and 真伪 is the pair genuine-or-fake.

Set it against 假 (jiǎ), which is the everyday word for fake and does stand alone: 这是假的. 伪 carries the extra weight of intent to deceive, and it is more formal. A toy sword is 假的, never 伪的. A forged passport is 伪造的.

The trap for English speakers is reaching for 伪 as a free adjective because dictionaries gloss it simply as "false". Say 假 in speech; keep 伪 for the compounds above and for written or legal register.