A men's shirt won't necessarily achieve a masculine look if it doesn't fit well. In the example below we have two figures wearing men's untucked casual button-down shirts.
The left figure's shirt is much too long, and covers their hips, making them look wider. The shoulders of the shirt are too wide, so they hang down too low on the arms. The figure on the right has the same body and is wearing the same pants, but their shirt fits differently.
To look more like the right-hand figure:
- Choose a shirt with a crisp or stiff fabric so the shoulders hold their shape.
- Hem the shirt so it falls at (or just below) the high hip at the side seams.
But don't go too far: a cropped top with high-waisted pants can also look more feminine.
As you may know, these points are only relevent in the context of current Western gendered fashion trends. Men have worn high-waisted pants and shirts covering their hips in various times and places. What you wear is ultimately up to you!