‘Well, Molly, I'm pretty sure this is a boggart,’ said Sirius, peering through the keyhole, ‘but perhaps we ought to let Mad-Eye have a shifty at it before we let it out — knowing my mother, it could be something much worse.’
Harry Potter and the Order of the Phoenix
I don't understand the phrase "have a shifty at". "Shifty" is actually an adjective word, but here it's used as a noun. I guess it probably means to let Mad-Eye take a look at it with his shifty eyes, but I don't know if it's a correct interpretation. How should we understand it here?