seat stay

Etymology
Possibly borrowed from sailing jargon where "stay" refers to a rope, wire, or rod which runs fore-and-aft along the centerline of a sailing ship from mast to deck, or to another mast, and serves to stabilize the mast.

Noun

 * 1) One of the two bicycle frame tubes running diagonally downward from the top of the seat tube to the rear dropouts.
 * The bike was not safe to ride as it had a bend in the seat stay

Translations

 * Dutch: staande achtervork
 * Finnish: takahaarukan yläputki; ylätakahaarukka