<[ Now I'm curious what the big Oxford Dictionary of the English
<[ Language says about 'blackleg' !

Okay, Robbie, my copy of the Oxford dictionary defines 'Blackleg, -legs'
as:

1.  A disease in cattle and sheep which affects the legs (better
black-legs),

2.  A turf swindler or swindler in gambling (origin lost),

3.  A local name of opprobrium for a workman willing to work for a
master whose men are on strike (also called black-neb),

4.  sc. a matchmaker (rare)

Sounds remarkably like the definition you posted of 'Jackleg' from your
friend's old dictionary.

So could it be that 'Black-leg' has been Americanized to 'Jack-leg'
(having no relation to the Oxford 'Jackleg'...the knife) to describe
strike-breaking workmen who, being not as good as the striking workers,
produced inferior results?

Or is it borrowing from the swindler definition: one who passes off a
so-called finished product that has been quickly and cheaply repaired
(such that the repairs will not last or that look awful).  Either would
fit the term's use at Piano-tuning school.  Neither describes a dog
with the weak bladder  :-))

Well, you're the Editor, so you get the last word (please!).

Musically yours,

Bill Wineburgh
http://members.aol.com/WWineburgh/musicbox.html

 [ It could be...!   ;-)   -- Robbie