Furtive and sly are both adjectives used to describe someone or something that is SNEAKY. They both imply that the person or thing is acting in a secretive manner, often with the intention of avoiding detection. Furtive is derived from the Latin word furtivus, meaning 'STOLEN, ' while sly is derived from the Old English word slīh, meaning 'CUNNING.' Both words can be used to describe someone who is trying to hide their actions or intentions.