It was the “N-word”, used by a passenger on the upper deck as we passed through an area where many of the population are of West Indian heritage, and it came as a jolt to the rest of us, who all involuntarily drew a sharp intake of breath.
He was a repulsively typical specimen of the sort of thick, yobbish lowlife with a brain the size of a pea, a foul mouth and an aggressive attitude.
True to his type, when several of us turned round in our seats to see who had just used this most racially offensive of words, we were treated to a volley of swearing.
The word he’d used came as such a verbal slap in the face because of its absence from the vocabulary of anyone with a sense of decency and fairness.
Its offensive counterparts aimed at other ethnic groups are similarly absent, which is something to be profoundly glad about.
And a key reason why everyday life has been largely scrubbed clean of these vile epithets was a ground-breaking piece of legislation enacted 50 years ago this week that deserves to be celebrated as a milestone in the history of our country.
The Race Relations Act, which came into force on December 8, 1965, set Britain on course not only to embrace the presence and talents of people from different backgrounds, but also set it on the road to being a kinder place.
It stood squarely in the honourable British tradition of fair play, and it is possible to draw a direct line from William Wilberforce’s tireless fight to abolish slavery to enshrining in law that a person should not be discriminated against because of the colour of their skin.
Yet the Act’s journey to the statute book had been bumpy. Even though it forbade discrimination on the “grounds of colour, race, or ethnic or national origins”, it stopped short of making it a criminal offence.
Harold Wilson’s Labour government had intended to do just that, but Conservative opposition saw the act watered down so that discrimination became a civil offence.
There were also gaps in the legislation. Penalties against employers who discriminated against job applicants on the grounds of race were absent, as were sanctions against local authorities who refused housing because of ethnic background.
Oddly, shops and private boarding houses were exempt from the Act, but hotels and restaurants were not.
Acts of discrimination included refusing to serve a person, an unreasonable delay in serving someone or overcharging.
There were undoubtedly flaws in the Act, but it was nevertheless an important change in the law because it sent out a clear and unambiguous message that discrimination was wrong, laying the foundations for all the equality legislation that has followed. Things did not change overnight. It took time, but gradually the Act’s aims were embraced by a majority of Britain’s people.
It was the start that was required and enshrined the spirit – as well as the letter – of how a modern and forward-looking country should treat everybody as equals.
And how it was needed. Those of us who grew up in the 1960s and 70s wince at the recollection of the things that were said and done, the casual racism that pervaded everyday life.
Some of it was a cultural hangover from the British Empire, which had crumbled in the 20 years leading up to the Act as countries threw off the shackles of rule from abroad and sought independence. Even though the empire was gone, one of its legacies was a leave a widespread sense of superiority over people from other lands, and in too many cases, an irrational hostility as well.
Even as the 60s turned into the 70s, with Britain supposedly more liberal than ever before in its outlook on everything from sexual freedoms to swearing on television, substantial sections of society remained stubbornly wedded to an offensive and suspicious view of non-white people.
Language that is completely unacceptable now was bandied about everywhere from playgrounds to pubs. Teachers stamped on it in schools, but they could do nothing about what the children were hearing at home.
Or what they saw on the television in the name of mainstream entertainment. Twenty million people a week watched The Black and White Minstrel Show on the BBC, with its cringe-inducing caricature of black people. It lasted until 1978 before being axed, but even then, like a reanimated corpse from a horror film, staggered around provincial theatres for a further decade.
And over on ITV, the hit comedy was the execrable and unfunny Love Thy Neighbour, which raised its laughs by aiming insults at black characters.
Against such a backdrop of popular culture, it was inevitable that insults were being hurled in real life too.
The memory of a shameful episode I witnessed has stayed with me for 40-odd years, of a black family being refused service in a Leeds fish and chip shop by the owner, who relished dishing up a helping of racial abuse instead. There must have been countless other cruelties and indignities just like it inflicted on people all over the country.
They could not happen now, and a key reason why was the marker laid down by the Race Relations Act.
It was one of the most socially significant pieces of British legislation of the 20th century, but more than that, it held a mirror up to society that made many realise their attitudes were unacceptable and unpleasant, and they needed to change their ways.
Society did change its ways, and we’re much the better for it. It is a measure of how far we have travelled that the word uttered on the bus now causes such offence, and how isolated and contemptible are those who would speak it.