What it means
A laughing stock is someone or something everybody's taking the piss out of because they've messed up, looked ridiculous, or become an easy target. It's not friendly laughing with you, it's the room pointing and having a field day at your expense. Can be a person, a team, a company, even a terrible outfit choice.
Usage examples
"I tripped over the mic at the awards do, now I’m the laughing stock of the whole office. Don’t worry, mate, half of them were trollied."
"He turned up to the wedding in the wrong suit and became the laughing stock of the whole family."
"If we miss this deadline we will be the laughing stock of the entire industry."
"After that horror presentation, the startup was the laughing stock of the whole conference by lunchtime."
"One dodgy tweet and suddenly he's the laughing stock of group chat and half the neighbourhood."
Where it comes from
The phrase comes from the old punishment stocks, those wooden frames used in public to hold offenders in place while crowds mocked them. So a laughing stock was literally a person stuck there getting laughed at by everybody. The modern meaning kept that same public-humiliation sting.