The whole point of thieves cant is that it seems like ‘normal’ speech to outsiders, though. It just hides illicit meanings within, while sounding like very bland/innocent common (or whatever is used as the mask). Undercommon is immediately noticeable as foreign and pointed out.

Time to Grandfather’s Axe that. Take the +1 head and put it on your shaft.

(I know the +1 makes it magical, so you can’t just ‘destroy’ the axe like that but shhh)