milk-curdling enzyme, 1897, from rennet (n.1) + -in (2).
Entries linking to rennin
rennet n.1
"inner membrane in the stomach of an unweaned calf or other animal," used for making cheese, etc.; also the mass of curdled milk found in the stomach, mid-15c., probably from an unrecorded Old English *rynet, related to gerennan "cause to run together," because it makes milk run or curdle; from Proto-Germanic *rannijanan, causative of *renwanan "to run" (from PIE root *rei- "to run, flow"). Compare German rinnen "to run," gerinnen "to curdle." Hence, "anything used to curdle milk."
-in 2
word-forming element in chemistry, usually indicating a neutral substance, antibiotic, vitamin, or hormone; a modification and specialized use of -ine (2).