guileful adj.

mid-12c., from Old French guile "deceit, wile, fraud, ruse, trickery," probably from Frankish *wigila "trick, ruse" or a related Germanic source, from Proto-Germanic *wih-l- (source also of Old Frisian wigila "sorcery, witchcraft," Old English wig "idol," Gothic weihs "holy," German weihen "consecrate"), from PIE root *weik- (2) "consecrated, holy."

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).