Protesters wear masks of the late Chinese Communist Party leader Mao Zedong during an anti-mainland tourist rally in Hong Kong's famous Mong Kok shopping district March 9. (Reuters/Tyrone Siu)

Most Americans may not know what it means to “take the piss,” a British phrase for saying or doing something mocking or contemptuous that’s aimed at aggravating somebody else. But in another former British colony the idiom may be rapidly acquiring a whole new level of aggression.

A new, social media-fueled controversy is aggravating tensions between Hong Kong and China, which has ruled the territory since 1997. Footage emerged online last week of an incident in which a toddler -- the child of mainland Chinese tourists -- was seen urinating in a Hong Kong street. In the video, an angry, hectoring crowd in Mong Kok, a packed shopping district popular with Chinese tourists, surrounds the family, causing the child to cry. The parents try to grab the memory card of someone in the crowd who had filmed the child. People in the crowd shout at the family in Cantonese, as the parents respond in Mandarin. The mother was reportedly arrested for allegedly slapping someone. (You can watch one bystander's clip here.)

The video went viral, playing to the longstanding frustrations of Hong Kong residents who resent the influx of mainlanders into their city. Hong Kong's commerce secretary used the occasion to urge residents to calmly teach their mainland brethren better manners, a statement that, while aimed at being conciliatory, smacked of condescension.

Every year, some 30 million to 40 million Chinese tourists visit Hong Kong, which maintains its own immigration and customs controls. In the past, Hong Kongers have rallied against all sorts of perceived Chinese invasions: from pregnant mainlanders taking up space in hospitals to Chinese shoppers buying up Hong Kong-produced infant formula (rather than risk potentially tainted supplies on the mainland).

Epithets have been traded. Some Hong Kongers refer to the mainlanders derogatorily as "locusts," while a Chinese professor triggered a ruckus in 2012 when he labeled Hong Kongers the "dogs" of the lapsed British empire.

China's netizens have also responded to the anti-mainland prejudices. According to Shanghaiist, an online campaign  kicked up in the wake of last week's toddler episode that called for more Chinese tourists to take their children to Hong Kong to pee in more streets. In response, some Hong Kong protesters mimicked going to the toilet atop of portraits of Chairman Mao.

The Global Times, a Beijing-based state newspaper, issued a stern editorial on the matter, condemning Hong Kong's coterie of "hooligans":

This handful of radicals in Hong Kong remind us of the rampant skinheads and neo-Nazis in Europe. Xenophobia is the cult of these groups. Their opinions have an effect on public opinion, but their actions will usually make trouble for mainstream society.

But it went on to add, perhaps more reasonably:

Hong Kong returned to China not long ago, which means it is still a young member in this big Chinese family.

It is unavoidable that some troublemakers will keep posing challenges to society.

But it must be noted that these people cannot be dealt with at the cost of the harmony of the whole nation.

Patriotism demands this massive country should have a certain tolerance for frictions within different groups.

The concern in Hong Kong over the mainland "threat" isn't just about urine in the streets and baby formula off store shelves. Hong Kong residents are seeing an erosion of political freedoms as Beijing's authority tightens over the restless special administrative region. In February, a prominent  journalist known for his work covering human rights and corruption was targeted in a mysterious knife attack. The violence raised fears of censorship and of darker forces from the mainland subduing Hong Kong's noisy public sphere.