The after-part of a ship, and therefore Shakspeare's term is simple enough for any but commentators. Henry V.'s fleet is sailing away:
"O, do but think,
You stand upon the rivage, and behold
A city on the inconstant billows dancing;
For so appears this fleet majestical,
Holding due course to Harfleur. Follow, follow!
Grapple your minds to sternage of this navy."