Green’s Dictionary of Slang

blag n.

[? SE blackguard]

1. [late 19C+] (also blag job, blague) robbery, often with violence, esp. of a bank or post office.

2. [1920s+] bag-snatching, watch-stealing.

3. [1930s+] a wages snatch.

4. [1940s+] (orig. UK Und.) a persuasive if lying story.

5. [1990s+] (Irish) an oppprtunity for crime.

In compounds

blag-merchant (n.) [merchant n.]

[1950s+] a pay-roll robber.