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5bang
to cut (as front hair) short and squarely across
[M-W's Dictionary]

What does squarely mean?
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Jackson66125bang
to cut (as front hair) short and squarely across
[M-W's Dictionary]

What does squarely mean?
Some ladies have the front hair half-way down the forehead (or lower), cut it a straight line parallel to the eyes. These are known as 'bangs'. Try googling for a picture - I'm sure you'll find one.

Editing to answer your specific question: squarely = straight.
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Is this dictionary definition suggesting that "to bang" as a verb includes this definition of cutting bangs? If so, you can forget it immediately. I have never, ever heard it used that way and if I walked in to my stylist and asked her to bang me, she would think I was making a rude sexual suggestion.
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Comments  
Grammar Geek...and if I walked in to my stylist and asked her to bang me, she would think I was making a rude sexual suggestion.
I don't know how others would take it but you really made me laugh.
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