A between maid (nickname tweeny, also called hall girl particularly in the United States) is a female junior servant in a large household with many staff. The position is largely defunct in the 21st century, as few households can afford great retinues of servants with the elaborate hierarchy of the past.
The term hall girl came from her chief duty, which was waitressing in the Servants' Hall. She was required to set the table and remove the dishes, as well as waiting at table. She may also have carried meals up to the head housekeeper, if, for example, that head of staff had breakfast or afternoon tea in her room(s).
The term between maid came from the fact that her duties came under the area of responsibilities of the housekeeper, butler and cook; if these individuals did not like one another the job of the between maid was a very difficult one.
In popular culture
Tweeny, from the film The Admirable Crichton and Dorothy, the still room maid in Gosford Park are examples of between maids. When the main characters in The Admirable Crichton were marooned on the proverbial desert island, the daughter of the house was disgusted that she would have to settle for Tweeny as her personal maid, calling her "unsuitable".