A frostie is a slang term for a frozen embryo. The term frostie is used most frequently in online fertility message boards.
FertilitySmarts explains Frostie
A cycle of in vitro fertilization (IVF) can include either a fresh embryo transfer or a frozen embryo transfer (FET). Embryos from a fresh IVF cycle can be frozen, or cryopreserved for future use. This is achieved through rapid freezing known as vitrification that does not harm the embryos.