slyest

adjective slier or slyer, sliest or slyest

1. Dialectal skillful or clever
2. skillful at trickery or deceit; crafty; wily
3. showing a secretive, crafty, or wily nature; cunningly underhanded
4. mischievous in a playful way; roguish

Origin: ME sley < ON slœgr, clever, cunning, lit., able to strike < base of slā & OE slean, to strike: see slay