Useful \Use"ful\, a. Full of use, advantage, or profit; producing, or having power to produce, good; serviceable for any end or object; helpful toward advancing any purpose; beneficial; profitable; advantageous; as, vessels and instruments useful in a family; books useful for improvement; useful knowledge; useful arts.

To what can I useful! --Milton.

Source: Webster's Revised Unabridged Dictionary (1913)