The phrase "get off your duff" is a call to action. The recipient of this exhortation is (literally or figuratively) sitting, unmoving, and is being asked to get off of his buttocks, as seen in Entry 2.
Definition of duff
(Entry 1 of 3) 1: a boiled or steamed pudding often containing dried fruit
2: the partly decayed organic matter on the forest floor
3: fine coal : SLACK duff
(Entry 2 of 3) : BUTTOCKS get off your duff
(Entry 3 of 3) British : INFERIOR, WORTHLESS
(Source: Merriam-Webster) The same site lists the etymology:
History and Etymology for duff
Noun (1) English dialect, alteration of dough
Noun (2) origin unknown
Adjective duff, noun, something worthless, from DUFF entry 1
The Online Etymology Dictionary agrees:
"buttocks, rump," 1830s, of unknown origin. The word had a variety of colloquial, slang, or provincial senses late 18c.-early 19c., some of them at least probably related: "dough," also "stiff flour pudding" (nautical, 1840); something worthless or spurious (1781).
This site has a discussion of the BE phrase up the duff. The accepted answer says that the OED says it is of Australian origin, and is a slang phrase for being pregnant. (BE speakers may recognize the pudding club and AE speakers would recognize a bun in the oven as euphemisms for pregnancy; this would fit well with Entry 1.)
A folk etymology could stab at dough or pudding referring to buttocks. Perhaps:
- The resting person is torpid from eating sweet pudding.
- The buttocks are associated with dough, either through shape or over-consumption.
- The buttocks are associated with inferiority or worthlessness.
And a folk etymology for the association with pregnancy:
- She is avoiding strenuous labor because of her bun in the oven by resting on her backside.
But these seem forced.
How did duff come to mean buttocks?