Arelith

It's always been dumb, and not representative at all of what a thieves' cant is. A cant of any kind is a spoken language of code/jargon created to exclude/mislead people outside of the group. We can see an example in 2nd edition D&D, even, in the form of planar cant from the Planescape setting.

Some Canny Cutter wrote:"That prime has the look of a gully and a real jinkscore. Better doughty up and help ourselves to his jinglings like any good knight of the post, then give him the laugh real quick-like."

prime=someone from the prime material
gully=a particularly clueless and gullible sod
jinkscore=like the above, but with a lot of money ("jink")
doughty up=put on a disguise
jinglings=coin purse
knight of the post=a thief
give him the laugh=give him the slip/run away

So basically this canny cutter (cutter=a particularly cunning or daring person) is discussing a potential mark to a fellow thief, but in a way the mark, being unfamiliar with planar cant as he is from the prime material, would not understand. That is what a cant is. It isn't any kind of sign language or sneezing in any particular way which somehow conveys how you feel about the weather we're having. Gangs of thieves should make up their own cants, which is generally how cants work. Gangs of criminals don't want potential rivals/other thieves to know what they're saying either, so some universal "language" of coughing and sneezing is silly, in my opinion.

Remove it and encourage people to form gangs and make up their own cants.

Actually I agree with this.

A mechanic for thieves to set up their own unique cant would be great too, but failing that removing it seems a good course of action.

Because very few rogues are actually thieves.

Approved.

Alternative:
Apply as mask to existing language as added complexity and alternate format.