1420, from Anglo-Fr.
oster (1292), from O.Fr.
oster "put out, keep off, remove, avert" (Fr.
ôter), from L.
obstare "stand opposite to, block, hinder," from
ob "against" +
stare "to stand," from PIE base
*sta- "to stand" (see
stet).
Ouster (n.) is first recorded 1531, noun use of Anglo-Fr.
ouster.