The term 'crafty fellow' can be summarized by two crossword solutions: KNAVE (5 letters) and slydog (6 letters). A KNAVE is an unprincipled, untrustworthy, or dishonest person. The term is derived from the Middle English word KNAVE, which means 'boy or servant.' A slydog is someone who is cunning and crafty, often in a negative way. This term is derived from the combination of the words sly and dog, which suggests someone who is sly like a fox and as loyal as a dog.