Comes from the old word "blackguard", a contemptible and untrustworthy person, which was pronounced "blaggard".
The sound of the last part "ard" became confused with the suffix "ard": meaning a person who does an action, as in drunkard. "Blag" becoming the contemptible untrustworthy action.
I first came across it as a
teenager working on a building site nearly 50years ago, when an old Irishman accused another of "
fucking blaggarding". I didn't know what it meant, but was obviously a bad thing to accuse someone of and not done lightly. These days it seems to have
lost its former vehemence and is used more in the manner of
petty criminals boasting of their offences.