My take is this...there was a VERY popular comic strip years ago called
"Sad Sack"  The format is quite similiar to "Beetle Bailey" in that the
"Sad Sack" is a private who is always on the short end of the stick.  It's
humour was a little grimmer, relatively speaking, that "Beetle Bailey",
and the unhappy person with the bad luck and dour face was a "Sad Sack". 
I assume it's origins are somewhere in the history of the American
Infantry, perhaps someone with more direct knowledge could help there.

I assume most people know that Sir Galahad of the Knights of the Table
Round is held as the mythical example of the perfect knight, gentlemanly,
manly, etc., etc., the usual "guy stuff" that goes with traditional social
posturing.

So...the juxtaposition of "Sad Sack Sir Galahad" might be of one who
aspired to greatness but falls mortally short, as he *is* mortal.  And
that, as you know, is one of Harry's central musical themes--we shoot for
heaven and end up trying to stave off hell.

poof...

If there's no Chocolate, there's no point....SnoopyFan