Dictionary.net

• In fort. an outwork or rampart raised in the main ditch immediately in front of the curtain, between two bastions. In its simplest form it consists of two faces forming with each other a re-entering angle; but generally it consists of three faces forming two re-entering angles, in which case it is called a double tenaille. Any work belonging either to permanent or field fortification, which, on the plan, consists of a succession of lines forming salient and re-entering angles alternately, is said to be a tenaille.

1899 - The american dictionary of the english language.
By Daniel Lyons