William Safire quotes me again today:
Few terms, however, are applicable to a sexually attractive person of either sex. Hottie is not spelled with a y because -ie, the lexicographer Grant Barrett informs me, “is a classic diminutive or hypocoristic ending used for terms of endearment” (and hypocoristic is the Greek-based word for “called by pet names”).
The part in quotations is indeed what I told him and his lexical assistant, Juliet Mohnkern, but what comes before I did not say, because both -y and -ie are classic diminutive and hypocoristic endings, with -y being more common.
One data point that Bill did not have room for which is worth noting is that both the Dictionary of American Regional English and the Historical Dictionary of American Slang include entries for hot up, indicating that their editors feel it’s common enough in North America to be considered fully naturalized. DARE includes a first cite of 1942. This, of course, doesn’t mean that British influence on the term isn’t newly reinforcing its presence in our vocabularies.