I have got a chubby

A half-decent hard-on that's just enough to make you proud but not good enough to actually do anything with. It's like when your mom says you're getting somewhere, but still needs a little more work.

A soft, flimsy hard-on that doesn't stand a chance against anything real. It's like trying to fight a dragon with a popsicle stick.

A hard-on that's just enough to make you think you're doing good, but it's not strong enough to get you anywhere. Like getting a B+ on a test and being proud of yourself.