Packing refers to the wearing of padding or a phallic object in the front of a person's pants or underwear, to give the appearance of having male genitals. Packing is commonly practiced by transmen (i.e. female-to-male transgendered or transsexual people). People who cross-dress as male also pack, as do drag king performers.
The object used for packing, sometimes called a "packer", may take the form of only a penis shape or may also incorporate a false scrotum and testicles. Packers may be used to make male clothing hang and move correctly. If a packer is not used, the shape of the clothing is visibly "empty".
The phrase "soft packing" refers to packing with a device that cannot be used for sexual penetration. Homemade packers may be very low-tech and consist of rolled-up socks, condoms filled with liquids or gel. There are many commercially available packers, which more realistically emulate the size, shape, color and texture of the male genitals. All soft packers are made of soft, flexible material and simulate a flaccid penis. The term "packer" most frequently describes the soft models not used for sex. A few packers incorporate a receptacle that is inserted under and against the wearer's urethra. The packer has a tube from the receptacle to the tip, which allows the wearer to urinate through the prosthetic.
"Packing hard" refers to packing with a device that may be used for sexual penetration. Some such packers are made of firmer material such as silicone, and are flexible enough for packing but firm enough for sexual penetration. Still others are made of even firmer material, and feature either an internal rod that stays in the shape in which it is bent, or an internal hinge at the base. These types allowing the packer to simulate a flaccid position (if not a flaccid feel), as well as a very firm, erect position. A colloquial term for packers which may be used for sex is "packing dildo". A term for the usage of these is "pack and play".
A slang term related to packing is "packing heat". This slang term originally meant "carrying a firearm", but later was used to describe MTF transsexuals who still have a penis. Like many terms in minority communities, this phrase may be used affectionately or jokingly among friends, as a way of reclaiming a phrase that is used as a pejorative.