Utterly knackered...AKA breathing out of your arse...refers to the fact that you're hanging on by the chinstrap of your helmet.

Got the covid-19 vaccine

British army slang to mean extremely tired or worn out. As if to be hanging from the chinstrap of ones helmet.

During the performance of oral sex on a female, a bearded male will place his chin between the vaginal lips and shakes his head back and forth rapidly and lightly to induce a pleasurable experience for the female.