It's a shirt from a German brand called BUMBUM (unless I'm mistaken, it doesn't exist any more - but the photos are from their instagram site).
Perfect for a sports session or even for leisure, as it's very flexible and stretchy.
It's a size M but I'd say it's more of an S because it's quite tight around the waist. It's perfect because it doesn't move, even when you raise your arms.
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