If something is strappier, it has more straps or stronger straps.
Strappier can make people think of safety, strength, or being ready for action.
Examples:
The new backpack is strappier than the old one.
I need a strappier bag for my books.
She bought a strappier dress for the dance.
I wish my shoes were strappier so they wouldn't fall off.
The superhero's costume is strappier than mine.
I want a strappier helmet for my bike.