helpful adj.

Old English help (m.), helpe (f.) "assistance, succor," from Proto-Germanic *helpo (source also of Old Norse hjalp, Swedish hjälp, Old Frisian helpe, Dutch hulp, Old High German helfa, German Hilfe), from the source of help (v.).

The use of help as euphemism for "servant" is American English, 1640s (originally in New England). Bartlett (1848) describes it as "The common name in New England for servants, and for the operatives in a cotton or woollen factory." Most early 19c. English writers travelling in America seem to have taken a turn at explaining this to the home folks.  

A domestic servant of American birth, and without negro blood in his or her veins ... is not a servant, but a 'help.' 'Help wanted,' is the common heading of advertisements in the North, when servants are required. [Chas. Mackay, "Life and Liberty in America," 1859].

But help also meant "assistant, helper, supporter" in Middle English (c. 1200).

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.).

1590s, from un- (1) "not" + helpful. Related: Unhelpfully; unhelpfulness.