Crossword Genius

Lethargic without an undergarment (9)

Ross

I believe the answer is:

shiftless

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This could be a double definition.

'lethargic' is the first definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'without an undergarment' is the second definition.
('undergarment' can be 'shift' (shift is a kind of undergarment) so maybe 'without an undergarment' can be ''shiftless')

Can you help me to learn more?

(Other definitions for shiftless that I've seen before include "Lazy - indolent" , "Like an automatic American car" , "lacking in initiative" , "Idle" .)

I've seen this clue in The Daily Express.
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