Intro

google

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early 19th century: abbreviation.

wiktionary

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Clipping of  introduction, from Latin intrōductiō(“lead-in, introduction”) – the abbreviation removes the second part of the compound; the first part ultimately from Proto-Indo-European *h₁énteros(“inner, what is inside”). The demoscene sense comes from the fact that they were originally prepended to pirated copies of computer games. 

etymonline

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intro (n.)

short for introduction, attested from 1923.