From wikipedia.org:
A jammer, also called a swim jammer, is a style of swimsuit worn by male swimmers, used mainly in competition to obtain speed advantages. They are generally made of nylon and lycra/spandex material, but may be made of polyester, and have a form fitting design to reduce water resistance. The materials usually dry fairly quickly. They provide moderate coverage from the mid-waist to the area above the knee, somewhat resembling cycling shorts or compression shorts worn by many athletes. They provide greater leg coverage than swim briefs and square leg suits, although they also have slightly more water resistance.
Example from swimoutlet.com:
What is the translation of jammers in French?
My findings:
Google Translate doesn't translate the term.
larousse.fr doesn't have the term.
Linguee doesn't suggest any interesting translations.
OQLF notes "L'emploi de l'emprunt jammer est déconseillé."