What does Boxing Day have to do with boxing?
Old English leðer (in compounds only) "hide, skin, leather," from Proto-Germanic *lethran (cf. Old Norse leðr, Old Frisian lether, Old Saxon lethar, Middle Dutch, Dutch leder, Old High German ledar, German leder), from PIE *letro- "leather" (cf. Old Irish lethar, Welsh lledr, Breton lezr). As an adjective from early 14c.; it acquired a secondary sense of "sado-masochistic" 1980s, having achieved that status in homosexual jargon in the 1970s.
a girdle of, worn by Elijah (2 Kings 1:8) and John the Baptist (Matt. 3:4). Leather was employed both for clothing (Num. 31:20; Heb. 11:37) and for writing upon. The trade of a tanner is mentioned (Acts 9:43; 10:6, 32). It was probably learned in Egypt.