c.1300, "bewilderment," from Old French perplexite "confusion, perplexity," from Late Latin perplexitatem (nominative perplexitas), from Latin perplexus "confused, involved, interwoven," from per- "completely" + plexus "entangled," past participle of plectere "to twine" (see complex (adj.)). From 1590s as "something that causes perplexity."
The geometric mean of the number of words which may follow any given word for a certain lexicon and grammar.