The term "get-up" is a noun used to connote bizarre or unacceptable attire of some kind. Generally it is used for more than one item of clothing: the entire set of clothing is the get-up.
Get-up is usually elaborate in some way. For instance, a Halloween costume can be called (a) get-up. It is a set of constructed clothing; it is deliberate, worn for a reason. It is different from everyday clothing in that it is such a construction. It takes more effort than street clothes; it has more thought behind it than "well, these are clean and they match." The thought process goes more along the lines of "hmm, this would represent me as n." The get-up changes one's appearance; to an extent, it changes or establishes one's visual identity.
Often one's get-up becomes an issue of contention. This depends on the get-up itself as well as the line of reasoning used to wear it.
Your mother hates most clothing that she might refer to as "get-up", although she is fine with formal costumes. She will make the exact statement, "you're not getting out of this house in that get-up." This house. That get-up. It is the same phrase her mother said to her.
Perhaps an example from the fine film Clueless might be in order.
"What is that?"
Note the use of "that". This is a condemnation in itself.
The get-up in question is a dress, chosen by the protagonist as her attire for a date. Now, how does she want to appear on this date? Attractive. Yes? Thus she has chosen this specific get-up in order to appear the most attractive possible--she wants to look more attractive than she does ordinarily. She is wearing her get-up in order to change herself and others' views of her.
The interesting thing about the term "get-up" is its connotation. The use of this term--a derogatory term--condemns the get-up, but more specifically the person wearing such get-up. Clothing is still establishing identity here. It may not be a harsh condemnation, as, for instance, in the case of a costume, where "get-up" might simply connote its elaborateness and the guts it might take to pull such a thing off. But even in this situation it might be used to ridicule. You might hear gossips sniping at someone's get-up across a crowded party. And you certainly hear your parents condemning your get-up, and through it, you.