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dead
studied
jailer
teeth
posture
An idiom is a form of expression in a general group or culture. Something like 'It's raining buckets.' This doesn't mean there are literal buckets falling it just means it's raining hard. You most likely will have to define what each idiom means.
the cave entrance
tree branches
fundamental principle
Based on the context provided, the most likely meaning of the underlined word is "accident."
nephew
reclining