「Meat with service provider and tonguing rod in mouth」would certainly feel slippery as if coated with OIL or GREASE, and may be why FAT also means the best that is obtainable: PHAT.
　While not always the healthiest portions to eat, body FAT remains「tasty with meat, or, the tasty meat」even today. While not generally known, in the past if not still today, many COSMETICS such as lipstick and rouge were initially manufactured using the FAT of animals, such as fat rendered from skinned muskrat carcasses: rats with musk glands.
　「A meaning clear and obvious, make known what was hidden」by「service provider, person sitting and gathering fluids for a brief time, repeatedly bobbing head of penis」describes a waxy, slippery substance generally found on the glans and inside the foreskin of a penis known as SMEGMA (can't say how it tastes, though).
　FAT stems from the folks who were forced to sit most of the time, with a lack of exercise due to having no-longer-integrated feet or a royal position, but nevertheless continued to eat fairly well and became obese (豕):「body meat of sitting person (concubine) with tonguing mouth」describes someone who only exercises the tongue, lips, and jaw muscles regularly and not the whole body, inevitably becoming FAT, a skilled sucker.
　「Body meat of sitting person with tonguing mouth」also aptly describes what today in American vernacular is referred to as a 'couch potato' munching chips and just being FAT…paradoxically however, potatoes are comprised primarily of starch carbohydrates and proteins, not fats, and contain many necessary nutrients, such as iron, magnesium, potassium, vitamin C, and dietary antioxidants. The epithet 'couch potato' seems to refer more to the dimpled bulbous shape of the tuber, rather than its nutritional content. Grease and oil from deep frying with sour cream and onion dip consumed with those ripple-cut taters don't help the fat situation though.
　「Rod in orifice of service provider meat」also may require some LUBRICATION, FAT, GREASE, OIL or a water-based equivalent if available, since the service provider may not naturally become too sexually excited by the client, or, an orifice without natural lubrication has been selected for provision of the services.
　「Meat with imperial decree」and becoming GREASED in some manner as per the rules to prepare for the lord's rod in orifice of service provider.
　「In traditional medicine in Europe, human fat was believed to have a healing magic significance until the 19th century. Many executioners recovered the fat called Armsünderfett or Armsünderschmalz (German for fat or grease from poor executed sinners) from the bodies of their executants and sold it. For some executioners the marketing of human fat was a major source of revenue. In traditional medicine many other parts of executed bodies as well as their fat were awarded a special action force, which evolved from a pagan sacrificial belief. The human fat was used to make ointments for treatment of various diseases such as bone pain, toothache and gout. It was also regarded as a panacea for particular diseases are associated with cachexia (e.g. tuberculosis). Also an analgesic effect in rheumatoid arthritis was given to human fat.」
　　[　https://en.wikipedia.org/wiki/Human_fat 2021-02-17　]　

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: fat; rouge (cosmetics); resin
Japanese: fat, tallow, lard, grease, resin, tar, nicotine, sleep, to become greasy, to become oily, to be loud and garish, adipose fin, fatty
Unihan extended: fat, grease, lard; grease
EDRDG: fat; grease; tallow; lard; rosin; gum; tar