“Girl inches” refer to when a girl thinks a guys dick is much bigger than it actually is. This is partly due to guys over exaggerating their size.
So when a girl sees a dick thats around 6 inches, she’ll think its 8 or 9.
Joe: How did it golast night with the skank you met?
Jim: Well it went ok I guess. I still had about 4 dry inches to go when she said it hurt.
Joe: Man 4 dry? that sucks dude.
Arbitrary (and usually incorrect) units of measurement used mostly by males. Derives from men overestimating their penis size. Dick inches are much shorter than actual inches. Hence a guy can claim to have a 9 inch penis when it is actually closer to 5-6 inches. The term "dick inches" is usually used when over-estimating non-penis measurements.
fellow one: "Finally! There's a parking spot!"
fellow two: "No way, man...You can't park within 30 feet of a stop sign."
fellow one: "There's plenty of room."
fellow two: "Yeah, only if you're measuring in dick inches."