An Etymological Dictionary of Astronomy and Astrophysics

1. General: Two items of the same kind; a pair; something that joins or connects two things together; a link.
   

2. A system of two equal and opposite, parallel, but not → collinear forces acting upon a body.
   

3. To fasten, join together; connect.

See also → coupled, → coupled system, → charge-coupled device, → coupling,
→ decouple, → galvanic couple, → Nasireddin couple, → pair; → parity, → thermocouple.

Etymology (PE): 1) joft “pair, couple,” Lori, Laki jeft, Qâyeni jof, Tabari jeft, Mid.Pers. yuxt “pair, couple,” Av. yuxta- “a team of horses,” from yaog- “to yoke, harness, put to; to join, unite,” infinitive yuxta, Mid.Pers. jug, ayoxtan “to join, yoke,” Mod.Pers. yuq “yoke,” cf. Skt. yugam “yoke,” Gk. zygon “yoke,” zeugnyanai “to join, unite,” L. jugare “to join,” from jugum “yoke,”
P.Gmc. *yukam, E. yoke; PIE *yeug- “to join.”

2. joftniru, from joft as above + niru, → force.
   

3. Joftidan, infinitive from joft, → couple; jafsar kardan infinitive from jafsar, → coupled,

• kardan “to do, to make.”