To deceive or attempt to persuade by presenting to know, do or be something more or else than a person actually does or is. Sometimes unsuccessfully.
Verb, noun, adjective and pronoun (separately or all together).
Situation: Person 1 gives a series of advanced lectures about the laws of things that don't exist or aren't here and concludes by stating that too much hot air has been spoken on the topic by other persons.
Person 2: "they blag a bit don't they".
Person 3: "what a lot of blag".
Person 4: "and after sitting through those interminably blag deliverances, the blag tells us this".
Situation: a Professor announces to students that he will only be lecturing for half an hour of each scheduled lecture (being 60 minutes each in the schedule), every week for the duration of the semester because he doesn't feel like it.
Person 2: "who the blag does this blagger think he's blagging. I'm getting a fees refund".
Person 3: "blag it. blag. total blagging blag".