People who can afford a sirvienta (lit. a maid) do not like referring to her as such. They prefer the annoyingly long euphemism la muchacha que nos ayuda (lit. the youngwoman who helps us), which is usually shortened to muchacha or, more frequently, to just chacha.
Some people use chacha to refer to questionable taste and manners. For instance, the sentence la novia de tu primo es una chacha (lit. your cousin's girlfriend is a maid) doesn't mean she actually makes a living cleaning other people's home but rather that she is a déclassé.
A very popular expression is se fue como las chachas (lit. he/she left like maids), which means the person left without any notice, which is rude.
Note: Do not confuse chacha with chacha music, which in Mexico is called cha-cha-chá and in Mars is called ri-ca-chá, according to the famous chacha tune.