A word used excessively by chicano gang members/bangers and chicanos in general with limited education(i.e. Mickey Dee's employees and target workers) who are from Southern California. This term has a variety of meanings like expressing relief and anger, but its main use is to replace NO in certain conversations.
Toker from Eastside 13: Ay foo, did you go to the Tierra Concert last night wit tu rucca? I heard it was firme homes.
Silent from Hillside 13: Nah Foo! I got busted again for selling fake Raider Jerseys in the back of my Chevy. La jura asked me if they was mine but I replied, "NAH FOO!"