a combining form meaning “long,” used in the formation of compound words: longicorn.

Origin of longi-

< Latin, combining form of longus long1; see -i-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Word Origin and History for longi-

word-forming element meaning "long," from Latin longi-, comb. form of longus (see long (adj.)).

Online Etymology Dictionary, © 2010 Douglas Harper