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Malinger
Negative Connotation
Using 'malinger' often implies a judgment that the person is intentionally and dishonestly avoiding responsibility. 
Often Undetectable
'Malinger' is used when the pretense is about something that's hard to verify, like a headache or fatigue. 
Not Always Illness
While commonly about fake illness, 'malinger' can also relate to feigning other conditions to avoid duties. 