lie, falsehood, deceive, pretend
僞 centers on presenting something false as true: lying, deceiving, pretending, or passing off a fake as genuine.
僞 combines 亻 (person) with 爲, which likely contributes both sound and the idea of making or doing. The character suggests a person fabricating something false.
A person 亻 beside 爲, a shape associated with making or doing, suggests a person making up a lie or pretending.
For ギ, imagine a person telling a lie with a big grin: the grin cues ギ, and the person is deceiving you.