Use the adjective halfhearted to describe something that's unenthusiastic, uninterested, or grudging. A pitcher's halfhearted throw of the baseball isn't too likely to end up being a strike.
When you're halfhearted about the book you're reading, you may put it down mid-way through and never pick it up again, and a halfhearted audition for a play isn't likely to earn you a starring role. This noun, sometimes spelled with a hyphen, half-hearted, comes from the idea that if only half your heart is invested in something, you're not very excited about it. In the seventeenth century, the adjective half-headed was also common, meaning "stupid."
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