Often but not specifically after consuming large quantities of alcohol.
Person at a party:
*downs an alcoholic beverage then lurches forward with their hand over their mouth*
Everyone else in the vacinity:
"DON'T BERTIE! DON'T BERTIE!"
Person talking about a party:
"...and he drank half a bottle of straight gin, then bertied in a flowerpot."
Person who feels sick:
"Eurghh i think i'm gonna bertie."
Reputed by John Sessions on an episode of QI to have come from a conversation he had with either Alec Guinness or Laurence Olivier?? with regards to a theatre company he had either worked with or was advising him (JS) to avoid.
(2) Who with?
(1) the SDT
(2) oh they're an absolute fleet of bertie's