NO MATTER WHAT THE BASIS FOR THE WORD, IT IS NOW IN COMMON USE IN PLUMBING. AND WHAT’S MORE, THERE ARE SEVERAL DIFFERENT TYPES/SUBSETS OF PIPE NIPPLES TO CONTEND WITH. A SHORT LIST INCLUDES: CLOSE NIPPLE, HEXAGONAL NIPPLE, LONG HEX NIPPLE, REDUCING NIPPLE, HOSE NIPPLE AND WELDING NIPPLE. I’M SURE THERE ARE MORE, BUT WE’LL STICK WITH THOSE FOR NOW. IN ITS MOST BASIC FORM, A NIPPLE IS A SHORT LENGTH OF PIPE WITH MALE PIPE THREADS AT BOTH ENDS FOR CONNECTING OTHER FITTINGS.