The spelling of the word "cattie" can be a bit confusing. When we look at the phonetic transcription in IPA, it is written as /ˈkæti/. The "a" sound in the middle is pronounced as "æ" which is a short vowel sound that is similar to the "a" sound in "cat". The double "t" at the end of the word indicates that the "i" in "cattie" should be pronounced as a short vowel sound as well. Overall, "cattie" is spelled with a double "t" to indicate the pronunciation of the word.
The word "cattie" has its origins in the Scottish language. It is derived from the Old Scots word "catty", which in turn is derived from the Latin word "captivus". The word "catty" was used in Scottish English to refer to a weight measurement, equivalent to around a pound and a half or roughly 680 grams. Over time, the term "cattie" came to be associated with the weight measurement, and it is still occasionally used in informal contexts in Scotland.