No'-As-Big-As-Medium-Jock-But-Bigger-Than-Wee-Jock-Jock is a Feegle of repute, and Rob Anybody's brother. He served the clan as apprentice Gonnagle under the guidance of William the Gonnagle. During their raid into Fairyland to rescue Tiffany's brother he recited terrible poetry to drive off attacking fey.
His name comes from the fact there are a large amount of feegles called Jock in the Chalk Hill Clan and thus need distinguishing from one another. This is a recurring theme in Pratchett, cf. Medium Dave Lilywhite who falls somewhere between Big Dave and Wee Davy in the (admittedly huge) criminal element of Ankh-Morpork.
"No'" being an approximation of "Not" when spoken with a faintly Scottish accent, and actually pronounced "Naw".