People who think it's either clever or funny (or both) to put a spurious and wrong Latin plural of a commonly used English word in brackets with a question mark after using the correct but mundane English plural.
Often applied to 'penises' (penii?), 'geniuses' (genii?), and so on.
Example spotted in the wild in today's Guardian:
'The briefest of searches reveals that there are people out there engaged in Tom Baker scarf knitalongs, swapping encouragement over those tricky colour-changing points at a woolly Dalek's "shoulders", sharing patterns for K-9 tissue holders, knitted Tardises (Tardii?) and even a truly resplendent version in cream and red wool of Peted Davison as the Fifth Doctor.'
Why, Lucy Mangan, why? You know as well as anyone that 'Tardis' is not and never has been a Latin word. It is and always has been an acronym for 'Time and Relative Distance* in Space'. It's an English word, and its plural is 'Tardises'. You had it right the first time and what's more, you knew you did. Why let yourself down like that?
* Er, or, you know, 'Dimensions'. (Thanks LizSara!) Hmm, I may have shot myself in the foot a bit there.