I agree, it sounds weird. I'd avoid it all together and use on-site.
Edit to add: or, as uncle-comment (?) pointed out with the Wikipedia article, on-premises software is a thing. On-prem might be a good compromise, depending on your penchant for shortening like that.
https://www.merriam-webster.com/dictionary/on%20premises
Premise (singular) is a presupposition. The premise is that we can get this done in 3 weeks.
https://www.merriam-webster.com/dictionary/premise
I agree, it sounds weird. I'd avoid it all together and use on-site.
Edit to add: or, as uncle-comment (?) pointed out with the Wikipedia article, on-premises software is a thing. On-prem might be a good compromise, depending on your penchant for shortening like that.