"Bash her up" – offensive or just an idiom?

My girlfriend and I were having an argument over something that her female friend had said about me, which I found rude and displeasing. My girlfriend responded to my complaint by saying "What do you want to do, bash her up?" She thinks this phrase is normal, but for me, there's something inherently wrong about it, especially given that the implied meaning is a guy (me) beating up a girl, and even if it was meant to be an ironic statement. Am I getting too sensitive over what is perhaps just a harmless idiom, or should I stick to my gut feeling?

I picked up my English in North America, whereas my girlfriend did in Malaysia/Singapore, and so there's a good chance that it might just be a cultural difference, but for me that doesn't change my feeling that it's offensive.

"Bash her up" alone is offensive or not depending on how it's used.

For example,

• if you complaints were so strong that your girlfriend interpreted your displeasure might result in physical harm to someone else her question might be valid.
• If your complaints were reasonable/rational and your girlfriend was being hyperbolic her question is over the top generating a "Why would you ever think I would harm someone?" kind of response - offensive.

Other examples:

• If I read a story about someone who hurt someone else I might say "That was terrible, he really bashed him/her up" and it would just be a description of what happened.
• I might crash my car and say "I bashed her up"

It's all about the context of the statement.