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.
1728 in legal sense, "extinguishment by absorption," originally of real estate titles, from merge (v.), on analogy of French infinitives used as nouns (see waiver). From 1889 in the business sense "extinguishment of a security for a debt by the creditor's acceptance of a higher security;" not common until c. 1926. General meaning "any act of merging" is by 1881.
"to rise from or out of anything that surrounds, covers, or conceals; come forth; appear, as from concealment," 1560s, from French émerger and directly from Latin emergere "bring forth, bring to light," intransitively "arise out or up, come forth, come up, come out, rise," from assimilated form of ex "out" (see ex-) + mergere "to dip, sink" (see merge). The notion is of rising from a liquid by virtue of buoyancy. Related: Emerged; emerging.
1967 (earlier narco, 1960), American English slang, shortened form of narcotics agent. It had been used 1955 for narcotics hospital, 1958 for narcotics addict. The senses and spelling have tended to merge with older but unrelated nark (q.v.).
1727, "resemblance of sounds between words other than rhyme," from French assonance, from assonant, from Latin assonantem (nominative assonans), present participle of assonare/adsonare "to resound, respond," from ad "to" (see ad-) + sonare "to sound" (from PIE root *swen- "to sound").
More specific sense in prosody of "rhyming or correspondence of accented vowels but not consonants" is from 1823. In 20c. the sense tended to merge with consonance in the notion of slant rhyme, off rhyme, but properly there is a distinction.
Assonance is the relationship between words with different consonants immediately preceding and following the last accented vowels, which vowels have identical sounds (hit/will, disturb/bird, absolute/unglued). Consonance is the relationship between words whose final accented vowel sounds are different but with the same consonant frame (truck/trick, billion/bullion, impelling/compiling, trance/trounce). [Miller Williams, "Patterns of Poetry"]