* From "spiffy": "Spiffy" was an early 20th-century slang word meaning "smart" or "stylish," often used to describe clothing. It's possible "spiffing" arose as an intensifier of "spiffy," emphasizing the excellence of something.
* From "spiff" meaning "money": In some circles, "spiff" was slang for a small bonus or payment. This meaning could have led to "spiffing" being used to describe something that was desirable or worthwhile, perhaps something worth paying extra for.
The origin of "spiffing" is a bit uncertain, but it's likely a combination of these two possible meanings. It gained popularity in the 1920s and 1930s, particularly in British English.
It's worth noting that "spiffing" is now considered somewhat dated and is mainly used for humorous effect.