Q: Which is the proper form: “for free” or “for nothing”?
A: They’re both OK now, though “for free” apparently arose in the 1940s out of a confused conflation of “free” and “for nothing.”
The American Heritage Dictionary of the English Language (4th ed.) describes “for free” as an informal usage, but Merriam-Webster’s Collegiate Dictionary (11th ed.) treats it simply as standard English.
The Oxford English Dictionary lists the expression (in its entry for the preposition “for”) as “Chiefly U.S.” but doesn’t suggest that it’s anything other than standard English. The OED defines it as “for no charge, without payment.”
The phrase “for nothing” is much, much older, dating back to Anglo-Saxon times, when it meant for no consideration, on no account, or by no means. The OED says those senses are now obsolete.
The expression took on its modern meaning of free or without charge in the 16th century. In Shakespeare’s The Tempest (1610-11), for example, Stephano says, “This will prove a brave kingdom to me, where I shall have my music for nothing.”