In German this literally translates to something like “cute sand man”. Because in German you can just add an " I" at the end of any noun to make it sound cute and small. So “Sandi” can be thought of as a kind of diminutive of the noun “Sand”, similar to “Bauchi” from “Bauch” which is stomach. The correct dimunitve of “Bauch” would be “Bäuchlein” but this refers more to the perceived size of the object.
I mean it only works with nouns that are not ending on vowels and also not every noun works good.
But I heard this from people all over Germany. Mostly in a mocking way, like someone was eating too much and complaining about stomach pain:
“Oh, tut dir der Bauchi weh?”
Other commonly used examples I can think of:
- Lurch - Lurchi (amphibian)
- Frisch - Froschi (frog)
- Hund - Hundi (dog)
- Mutter/Vater/Oma/Opa - Mutti/Vati/Omi/Opi (mom, dad, grandma, grandpa)
I’ve only ever really heard it for a handful of very specific nouns, like Maus -> Mausi as an affectionate nickname
