Middle English pal, from Old English pæll "rich cloth or cloak, purple robe, altar cloth," from Latin pallium "cloak, coverlet, covering," in Tertullian, the garment worn by Christians instead of the Roman toga; related to pallo "robe, cloak," palla "long upper garment of Roman women," perhaps from the root of pellis "skin." The notion of "cloth spread over a coffin" (mid-15c.) led to figurative sense of "dark, gloomy mood" (1742). The earlier figurative sense is "something that covers or conceals" (mid-15c.).
"become tiresome or insipid," 1700, a surviving transferred or figurative sense from the earlier meaning "become faint, fail in strength," from Middle English pallen (late 14c.), which is apparently [OED] a shortened form of appallen "to dismay, fill with horror or disgust" (see appall). Related: Palled; palling.