From Wikipedia-
 
...The origin of the term "heavy metal" in a musical context is uncertain. The phrase has been used for centuries in chemistry and metallurgy, where the periodic table organizes elements of both 
light and 
heavy metals (e.g. uranium). An early use of the term in modern popular culture was by 
countercultural writer 
William S. Burroughs. His 1962 novel 
The Soft Machine includes a character known as "Uranian Willy, the Heavy Metal Kid." Burroughs's next novel, 
Nova Express (1964), develops the theme, using 
heavy metal as a metaphor for addictive drugs: "With their diseases and orgasm drugs and their sexless parasite life forms—Heavy Metal People of Uranus wrapped in cool blue mist of vaporized bank notes—And The Insect People of Minraud with metal music."
[55]
Metal historian 
Ian Christe describes what the components of the term mean in "hippiespeak": "heavy" is roughly synonymous with "potent" or "profound," and "metal" designates a certain type of mood, grinding and weighted as with metal.
[56] The word "heavy" in this sense was a basic element of 
beatnik and later countercultural 
slang, and references to "heavy music"—typically slower, more amplified variations of standard pop fare—were already common by the mid-1960s. British psychedelic art experimenters 
Hapshash and the Coloured Coat released a record in 1967 titled 
Featuring the Human Host and the Heavy Metal Kids. 
Iron Butterfly's debut album, released in early 1968, was titled 
Heavy. The first recorded use of "heavy metal" is a reference to a motorcycle in the 
Steppenwolf song "
Born to Be Wild", also released that year:
[57] "I like smoke and lightning/Heavy metal thunder/Racin' with the wind/And the feelin' that I'm under." A late, and disputed, claim about the source of the term was made by 
"Chas" Chandler, former manager of the 
Jimi Hendrix Experience. In a 1995 interview on the 
PBS program 
Rock and Roll, he asserted that heavy metal "was a term originated in a 
New York Times article reviewing a 
Jimi Hendrix performance," in which the author likened the event to "listening to heavy metal falling from the sky." A source for Chandler's claim has never been found.
The first documented use of the phrase to describe a type of rock music identified to date appears in a review by 
Barry Gifford. In the May 11, 1968, issue of 
Rolling Stone, he wrote about the album 
A Long Time Comin' by U.S. band 
Electric Flag: "Nobody who's been listening to 
Mike Bloomfield—either talking or playing—in the last few years could have expected this. This is the new soul music, the synthesis of white blues and heavy metal rock."
[58] In January 1970 
Lucian K. Truscott IV reviewing 
Led Zeppelin II for the 
Village Voice described the sound as "heavy" and made comparisons with 
Blue Cheer and 
Vanilla Fudge.
[59] Other early documented uses of the phrase are from reviews by critic 
Mike Saunders. In the November 12, 1970, issue of 
Rolling Stone, he commented on an album put out the previous year by the British band 
Humble Pie: "
Safe as Yesterday Is, their first American release, proved that Humble Pie could be boring in lots of different ways. Here they were a noisy, unmelodic, heavy metal-leaden ****-rock band with the loud and noisy parts beyond doubt. There were a couple of nice songs...and one monumental pile of refuse." He described the band's latest, 
self-titled release as "more of the same 27th-rate heavy metal crap."
[60] In a review of 
Sir Lord Baltimore's 
Kingdom Come in the May 1971 
Creem, Saunders wrote, "Sir Lord Baltimore seems to have down pat most all the best heavy metal tricks in the book."
[61] Creem critic 
Lester Bangs is credited with popularizing the term via his early 1970s essays on bands such as Led Zeppelin and Black Sabbath.
[62] Through the decade, 
heavy metal was used by certain critics as a virtually automatic putdown. In 1979, lead 
New York Times popular music critic 
John Rockwell described what he called "heavy-metal rock" as "brutally aggressive music played mostly for minds clouded by drugs,"
[63] and, in a different article, as "a crude exaggeration of rock basics that appeals to white teenagers."
[64]
Coined by 
Black Sabbath drummer, 
Bill Ward, "downer rock" was one of the earliest terms used to describe this style of music and was applied to acts such as Sabbath and 
Bloodrock. 
Classic Rock magazine described the downer rock culture revolving around the use of 
Quaaludes and the drinking of wine.
[65] Later the term would be replaced by "heavy metal."
[66]
The terms "heavy metal" and "
hard rock" have often been used interchangeably, particularly in discussing bands of the 1970s, a period when the terms were largely synonymous.
[67] For example, the 1983 
Rolling Stone Encyclopedia of Rock & Roll includes this passage: "known for its aggressive blues-based hard-rock style, 
Aerosmith was the top American heavy-metal band of the mid-Seventies..."