pronoun
such-and-such a place
Used to refer to a place whose name is not specified, similar to 'such-and-such a place' or 'somewhere or other'. Often used when the exact location is unknown or unimportant.
See also: 何処其処 (どこそこ)
彼はどこどこの出身だと言っていたけど、忘れてしまった。
He said he was from such-and-such a place, but I forgot.
どこどこの駅で乗り換えるように言われた。
I was told to change trains at such-and-such a station.
どこそこ is similar but often implies a more specific but unnamed place, while どこどこ is more vague and general.
Reduplication of どこ (where), forming an indefinite pronoun meaning 'some place or other'.