• (RP) IPA: /ˌfɪfθ ˈʍiːl/, /-ˈwiːl/
• (GA) IPA: /ˌfɪfθ ˈʍil/, /-ˈwil/

fifth wheel (plural fifth wheels) (chiefly US)

1. (road transport) A type#Noun|type of trailer hitch#Noun|hitch, which consists of a horseshoe-shaped#Adjective|shaped plate#Noun|plate on a multidirectional pivot#Noun|pivot, with a lock#Verb|locking pin#Noun|pin to couple#Verb|couple with the kingpin of a truck#Noun|truck trailer.
   1. In full, fifth-wheel trailer: a large caravan or travel#Noun|travel trailer that is connected to a pickup truck for towing#Noun|towing by a hitch similar to the one described in sense 1 located in the center#Noun|center of the truck's bed#Noun|bed.
2. (road transport, historical) A horizontal wheel#Noun|wheel or segment#Noun|segment of a wheel above the front#Adjective|front axle and beneath the body of a carriage, form#Verb|forming an extended#Adjective|extended support#Noun|support to prevent it from overturn#Verb|overturning.
3. (idiomatic, informal) Anything superfluous or unnecessary.

   Synonyms: spare tool, third wheel

   I felt like a fifth wheel when both of them started giggling and making out during dinner.

   • [1902 August 21, Henry James, chapter XVII, in The Wings of the Dove, volume II, New York, N.Y.: Charles Scribner’s Sons, OCLC 688890 ↗, book sixth, page 6 ↗:

     Compressed and concentrated, confined to a single sharp pang or two, but none the less in wait for him there on the Euston platform and lifting its head as that of a snake in the garden, was the disconcerting sense that "respect," in their game, seemed somehow—he scarce knew what to call it—a fifth wheel to the coach.]

• French: sellette
• German: Sattelkupplung
• Spanish: quinta rueda

• French: cinquième roue du carrosse
• German: fünftes Rad am Wagen
• Italian: ultima ruota del carro
• Portuguese: quinta roda, peso morto
• Russian: (как теле́ге) пя́тое колесо́
• Spanish: quedarse tocando el violín