The phrase "sleeve opening" refers to the part of a garment, like a shirt or a jacket, where your arm goes through. It is the opening or the hole through which you put your arm into the sleeve. Full definition
Stay sassy in this long sleeve open shoulder top with a cross detail and a ruffle along the shoulder line. (sophieandtrey.com)
A simple short sleeves open cardigan featuring a lightweight black and white striped material. (sophieandtrey.com)
It felt like I was dancing while wearing this blouse because the pleated sleeves opened up with every movement and brought even more dimension to the look. (mywhitet.com)