Definition of «left thumb»

The phrase "left thumb" refers to the thumb on your left hand, which is the finger located on the side opposite your right hand.

That way, leaving the thumb on the button does nothing. (blog.the-ebook-reader.com)
The neutral positions should be: analog for left thumb, and buttons for right thumb. (gonintendo.com)
Unless you have an extra left thumb lying around, you'll have to stop moving entirely to hit the d - pad. (xblafans.com)
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