Casual, often derogatory term for lewdness or a lecherous person. Can be used as a noun or adjectival noun (e.g., 助兵衛な人).
See also: スケベ (すけべい)
Watch out for that old guy; he's a lecher.
Please stop with the lewd talk.
エロい is a more general adjective for 'erotic' or 'sexy', while 助兵衛 specifically implies a lecherous character or behavior.
Standard kanji spelling for this word.
The exact derivation is uncertain; the kanji 助兵衛 are ateji, and the word is thought to have originated as a slang term in the Edo period.