A European dialect word
for 'Chicken or Hen', but was originally a word
/sound used when 'calling chickens (i.e. biddybiddybiddy).
It's quite uncommon
to hear it now, but it's an old (mostly British colloquialism) to refer to old women (of a certain disposition) as 'birds' or 'hens'.
Subsequently, the term 'Biddy' became a term used to refer to an old women, generally one who is fussy or garrulous.
Mrs. Charleton really has a lot to say about her neighbours, doesn't she.
that too. What an old biddy.