insider n.

late 14c., ynneside "interior part (of the body)," compound of in (prep.) + side (n.). General sense "inner side or part (of anything)" is from c. 1500.

The adjective sense "being on the inside" is from 1610s, from the noun. It began to be used in slang c. 1900 in reference to the supposed real facts or situation that only an insider would know. Inside man is from 1911 (originally in reference to workers used by management to sniff out union activity); inside job "robbery, espionage, etc., committed by or with the help of a resident or servant of a place" is attested by 1887, American English (also, late 19c., early 20c., "indoors work").

The figurative inside track "advantage" (1854) however is a metaphor from horse racing (1830); inside lanes are shorter than the outer ones on a curved track. Adverbial use in American English inside of (in reference to time) is from 1839.

English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).

Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.

The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).