What does LYG in jewelry mean. ie. 10K LYG 8 15.8 GRMS. 90TW?

In the context of jewelry markings, "LYG" typically stands for "Layered Yellow Gold." This indicates that the piece is made of a base metal coated with a layer of yellow gold. In the example you provided (10K LYG 8 15.8 GRMS. 90TW), it suggests that the jewelry is 10 karat yellow gold layered over another metal, weighs 15.8 grams, and has a total weight of 0.90 carats of gemstones.