sick = poor health, infirm = very poor health, moribound = in dying condition
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silly = thoughtless, lacking judgement,
daft = foolish, crazy
Meanings of the given options
frown:
1. (noun) wrinkling forehead and eyes together to express displeasure
2. (verb - intransitive, figurative) To manifest displeasure
fawn:
1. (noun) A young deer
2. (verb - intransitive) To exhibit affection or attempt to please; to seek favour by flattery and obsequious (extremely attentive) behaviour
vein:
1. (noun) A blood vessel transporting blood from the capillaries to heart
2. (noun - figurative) A style, tendency or quality; a topic of discussion, a train of association of thoughts, emotions, etc.
vain:
1. (adjective) Having no real substance, value or importance; empty; void; worthless; unsatisfying
2. (adjective) Effecting no purpose; pointless, futile.
3. (adjective) Showy, ostentatious.
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So, the most suitable word that can fit the given analogy is vain = pointless, futile.