This is the cuter, more casual version of "Ate." It's what you call your older sister when you're about to ask for a favor ("Ateng, pautang..."). It can also be used in a slightly playful or even gossipy tone, like what your tita might say: "Nakita mo ba si Ateng, ang ganda ng bag!" It has a certain "palengke chic" or friendly neighborhood chismosa vibe.