before vowels nitr-, word-forming element used scientifically and indicating nitrogen, nitrate, or nitric acid; from Greek nitron (see nitre).

a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].

before vowels non-, word-forming element indicating "nine," from combining form of Latin nonus "ninth," contracted from *novenos, from novem "nine" (see nine).

word-forming element meaning "disease," from Greek nosos "disease, sickness, malady," a word of unknown origin.

before vowels nyct-, word-forming element meaning "night," from Latinized form of Greek nykti-, a combining form of nyx "night" (see night).

word-forming element, principally in the science, forming names of disciplines describing natural laws or scientific methods; from Greek -nomia "method," -nomos "managing," nomos "law, usage, custom," all from the verb nemein "to manage," from PIE root *nem- "assign, allot; take." 

abbreviation of Latin nota bene "note well," 1670s.

1885 as abbreviation of New English Dictionary, the short form of the name under which the Oxford English Dictionary was published (A New English Dictionary on Historical Principles). It began to appear in volumes in 1884; in 1895 the title Oxford English Dictionary first appeared on the covers.

abbreviation of no good, attested from 1838; variant n.b.g. for no bloody good is by 1903.

chemical symbol for sodium, from Latin natrium (see sodium).