Lacking meaning, purpose, or good reason; or unconscious and unable to feel or think.
From 'sense' (from Latin 'sensus' meaning 'feeling') plus the suffix '-less' meaning 'without.' Literally means 'without sense.'
The word has two opposite meanings that somehow coexist: 'senseless violence' means unreasonable, but 'knocked senseless' means literally unable to sense. English lets the same word mean both 'illogical' and 'unconscious' because both involve missing something.
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