strap-on

1. not comparableThat can be attached using one or more straps.
2. not comparablespace flightOf a booster rocket, that is mounted on the outside of a launch vehicle (typically around the first stage) to provide an additional boost at lift-off, ultimately separating after the first few minutes of ascent.

plural strap-ons

1. space flightA strap-on booster.

   As a standalone booster, according to Pentagon officials, it could be used to put the smaller reusable spaceplane into orbit. But its more innovative use will be as strap-ons for the larger shuttle.

   1987 March, James Schefter, “The Other Shuttles”, in Popular Science, volume 230, number 3, →ISSN, page 76

2. informallifestyleAn artificial phallus attached with a strap, usually a dildo.
3. informalentertainmentSynonym of keytar (“keyboard supported by a strap around the neck and shoulders”).

   If you want a portable keyboard, a strap-on could be the answer. Strap-ons have been around since Billy Preston and Edgar Winter used to sling Univox electric pianos around their necks in the pre-MIDI days of the early ’70s.

   1988, Helen Casabona, David Frederick, “Alternate Controllers”, in Brent Hurtig, editor, Advanced MIDI Applications: Computers, Time Codes, and Beyond (Keyboard Magazine Library for Electronic Musicians), Van Nuys, CA: Alfred Publishing, →ISBN, pages 34–35

   Synonyms: keytar