"This is my seventh skirt."

[Mirage20]

Why is etekliğim not interchangeable with eteğim?

[AlexinNotTurkey]

I think I had answered you before, but to the best of my knowledge "eteklik" is something having to do with a skirt. It isn't a skirt itself. I have never heard that word over y 13 months in Turkey :)

[Selcen_Ozturk]

it certainly is NOT used in modern Turkish. I have never heard it in my life. It is sometimes used for the piece of cloth you intend to use to create a skirt

edit: TDK BTS (büyük Türkçe sözlük) includes almost all the words in the language, although some are archaic or local.

[sumermann]

According to TDK(Turkish Language Institute) Eteklik and etek are same words. However, you can use one instead of another. After all eteklik doesn't prefer to use much in Turkey.

[AliTorpheh]

Why "...etegi dir" is not correct?

[Bailador92]

"Bu benim yedinci eteğiMdir." would be correct. You should add the possessive.

You can use "eteğidir" for "her".
e.g. This is her seventh skirt.
Bu onun yedinci eteğidir.