Well, this is a confusing one! It currently means shockingly bad, but originally meant outstandingly good.

• Oxford: outstandingly bad, shocking; (archaic) remarkably good
• Collins: very bad indeed

It comes from the Latin egregius, meaning illustrious, distinguished, eminent, exceptional, or literally ‘standing out from the flock’. Oxford says the current meaning that indicates disapproval probably began as an ironic usage at some point in the 16th century.