Skulduggery

This word remains an etymological mystery. It describes dishonest, underhand or wicked behaviour and first appears in this sense in America during the mid-19th century. Before this, from the early 18th century, there was a Scottish dialect word, skulduddery, which meant fornication or adultery. The theory, and it is only a theory, is that skulduggery evolved from this old Scottish word. The OED’s judgment, however, is origin unknown.