This phrase originated in the English music halls before World War I, and was used as a general purpose catchphrase.

Usually used to mean either 'nonsense' or a general term to mean stuff, much in the same vein as 'thingummybob' or 'doohickey'. In recent times has taken on an air of sexual innuendo as in 'Do you fancy a bit of How's your father ?'. See also a bit of the other.