1630s, "to plunge or sink in" (to something), a sense now obsolete, from Latin mergere "to dip, dip in, immerse, plunge," probably rhotacized from *mezgo, from PIE *mezgo- "to dip, to sink, to wash, to plunge" (source also of Sanskrit majjanti "to sink, dive under," Lithuanian mazgoju, mazgoti, Latvian mazgat "to wash").

Intransitive meaning "sink or disappear into something else, be swallowed up, lose identity" is from 1726, in the specific legal sense of "absorb an estate, contract, etc. into another." Transitive sense of "cause to be absorbed or to disappear in something else" is from 1728. Related: Merged; merging. As a noun, from 1805.

"act of waiving," 1620s (but in modern use often short for waiver clause); from Anglo-French legal usage of infinitive as a noun (see waive). The specific sports club sense of waivers is recorded from 1907, in baseball (waiver clause is by 1894).

Other survivals of noun use of infinitives in Anglo-French legalese include disclaimer, merger, rejoinder, misnomer, ouster, retainer, attainder.