It's a British term that means to pretend to be ill in order to avoid work or school.
Here are some other idioms that convey a similar meaning:
* "To call in sick" - This is a more neutral term, but it can be used to describe faking an illness.
* "To pull a sickie" - This is another British term that is synonymous with "to fake a sickie".
* "To play hooky" - This usually refers to skipping school, but it can also be used to describe avoiding work.
* "To take a sick day" - This is a more formal way of saying that you're taking a day off work because you're sick.
It's important to note that while these idioms are often used in a lighthearted way, faking an illness is generally considered unethical.