The Joke:
My girlfriend says I'm the cheapest man in the world....I'm not buying it!
The expression I'm not buying it" normally means: “I don’t believe it.”
But in this joke, the guy is called cheap (someone who hates spending money).
So when he says:
“I’m not buying it”
It has two meanings at the same time:
1- “I don’t believe that.”
2- “I’m not going to buy anything.”
So basically: She says he’s cheap…
And he responds in a way that sounds like he’s being cheap again 😂