whack 'whacked'

[Laurence Horn]

>>NSOED2 thinks "jack (off)" 'masturbate' is a development from the noun
>>"jack" 'man', itself from the proper name.  it thinks "wank (off)" is
>>of unknown origin.
>
>Of course you're never provably wrong in choosing "unknown origin". I doubt
>"jack off" from "jack" = "man" (where are "man off", "bloke off", "guy
>off", "tom off", "joe off"?); I suspect "jack" is basically like "jack up
>the car to change the tire" (if there's any documentary evidence, of
>course, that trumps my idle notions).
>

I was wondering whether "jack off" might have originated as an
alteration of "jerk off", where the meaning appears to be derived
from the motion involved, but while HDAS suggests the two are indeed
alternants, the former does have a slightly longer pedigree, and the
OED clearly assumes "jack off" does indeed originate as Doug
suggests--not with car tires so much as the jacking (up) of large
objects in general, which predated horseless carriages as such.
Train cars could be jacked up, for example:

1885 Pall Mall G. 20 Mar. 6/1 To 'jack-up' a seven-ton engine and
replace it on the rails.

and the same general entry that gives this as the first sense (JACK,
v. 1) gives "jack off" under sense 5, with an apposite quote from
Orwell, albeit postdating HDAS's first cite [from a venery lexicon]
by 19 years:

1935 'G. ORWELL' Clergyman's Daughter ii. 109 Flo and Charlie would
probably 'jack off' if they got the chance of a lift.

Larry Horn