in law, "ejection from property, eviction by judicial process," 1530s, noun use of Anglo-French ouster "remove, evict" (see oust). For other such usages, see waiver. General sense of "dismissal, expulsion" is by 1961.
early 15c., ousten, "eject, dispossess," from Anglo-French oster,ouster (early 14c.), Old French oster "remove, take away, take off; evict, dispel; liberate, release" (Modern French ôter), from Latin obstare "stand before, be opposite, stand opposite to, block," in Vulgar Latin, "hinder," from ob "against" (see ob-) + stare "to stand," from PIE root *sta- "to stand, make or be firm." Related: Ousted; ousting.
"act of waiving," 1620s (modern usage is often short for waiver clause); from Anglo-French legal usage of infinitive as a noun (see waive). Baseball waivers is recorded from 1907. Other survivals of noun use of infinitives in Anglo-French legalese include disclaimer, merger, rejoinder, misnomer, ouster, retainer, attainder.