The concealed strength meaning of sandbagging that you mentioned is also
used in poker. When you either pass or make a small bet with a very
strong hand, hoping to lure the unwary into making a foolishly large
wager, you're sandbagging. This is very much the spirit of our
Sandbaggers. Didn't Willie, or someone, make a poker reference somewhere?
As a weapon, a sandbag has the virtue of not leaving a visible mark,
which is much like Willie's comment to C at the end of Unusual Approach
that Neil won't let Jeff Ross get away with his manipulations, "but he'll
never know he didn't get away with it."
The OED quotes some 1890's usage as:
"The sandbagger and blackmailer"
"He had not been sandbagged, or buncoed"
"The predatory rich do not shrink even from using the sandbag
and the revolver -- of course by deputies."
These examples are from American sources, but they don't go far astray
from the show.
Richard Anderson