Done without much enthusiasm or effort; lacking conviction or full commitment.
From 'half' + 'hearted' (from 'heart' meaning courage, commitment). Since medieval times, the 'heart' symbolized commitment and passion, so 'half-hearted' meant only half-committed.
The metaphor is brilliant—if your heart (your core commitment) is only 'half' there, you're basically sleepwalking through the task, which is why it perfectly describes lazy or reluctant effort!
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