The water supply tubing for a lavatory or a sink. Made of chrome plated brass, brass, copper, or plastic it has been given it's nickname because of it's similarity in appearance to male genitals.
"You want to attach your pecker head to the angle stop with a compression nut and ferrule."
In the plumbing field a nipple is a predetermined short length of pipe usually NPT(National Pipe Thread). A NPT threaded nipple can be made of almost any material and can be in sizes ranging from 1/16" to 24" NPS(Nominal Pipe Size). The plumbing nipple gets it's nickname because of it's similar function to that of a female nipple in humans and animals. It's usually a small protrusion which emanates fluid or gas.
"You want your nipple an 1 5/8" off of the finish wall in this situation."
When a plumber snakes a live sewer line to clear a stoppage and is sprayed in the face with "solids" when removing the snake from the line.
Alternate Definition: Any situation in which "solid wastes" are placed on the face.
"I pulled this toilet to snake the flange and totally got shit faced afterwards. It's not my day today."