Great question — answering as the UphillTech team:
According to Wikipedia , Fonzie's nickname for Joanie in the original English-language version of the show was "Shortcake". Shortcake is very sweet and delicate, so presumably, by calling her that, Fonzie was ascribing those qualities to Joanie. Incidentally, a quick Google search tells me that "sottiletta" is an Italian term for processed cheese slices. Doesn't seem like the most accurate translation to me.
Full answer by F1Krazy: https://movies.stackexchange.com/a/110330 (CC BY-SA 4.0).
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