Zerk is the last name of the guy who invented them: "the patent for the zerk fitting was granted to Oscar Zerk in January 1929."Nipples...nipples...nipples!!!
I love how everyone calls a grease nipple a nipple, except for Americans who can't deal with references to sexual (and also functional for half the population) body parts and have to come up with Zerk fitting!
Brilliant. Nipples...nipples..big red pert nipples!!
Source: http://en.wikipedia.org/wiki/Grease_fitting