From Hull AWE
The pistachio nut, and tree from which it comes, is pronounced as it is written - but in different languages, the rules are different, producing different results.
- In English, where the '-h-' as a rule makes the '-c-' soft, pistachio is usually pronounced 'pist-AtCH-i-oh' (IPA: /pɪs ˈtɑːʃ i əʊ/). The plural recommended by on-line Fowler is pistachios.
- In Italian, which has given the form to English (the word first entered our language through the French pistace with a soft '-c-'; both French and Italian derive through the classical Latin pistacium and Greek πιστάκιον 'pistakion', with a hard '-k-' from Persian), the '-h-' has the opposite effect: it makes the '-c-' hard. The Italian pronunciation is 'pist-AHK-i-oh' (IPA: /pist ˈɑːk ɪ o/). (The Italian plural is pistacchi, 'pist-AHK-ee', /pist ˈɑːk iː/.)
Unless you want to sound knowledgeable or affected, AWE recommends that you use the English pronunciation and plural. (This is different from the recommendation for radicchio.)