A packer is a prosthetic flaccid penis.
Probably derived from the term ‘Packing Heat’ (possessing a weapon/possessing a penis), a packer is an artificial dick that is not used for penetration (eg. like a strap-on) but rather to be worn in all contexts (not exclusively sexual) under clothes to give the appearance of the wearer to be… packing heat.
Packing is often practiced by trans men (Female-to-Male Transgender).
Packers come in a range of different materials and some are even firm enough to be used for penetration.