user150: According to the following wiki link,

The unofficial title, "Super Grandmaster", is often used by players to refer to those with a 2700+ rating to distinguish the most serious world champion contenders

When was this term first used and by whom?

user167: As pointed out in the comments, it's nigh impossible to pin down the very first usage, but since you're after a rough idea, I'll get the ball rolling with an initial answer of the oldest definite use I'm aware of; it's from within the last 25 years, so maybe others can improve on this with better answers.
In 1990, there was a tournament held in New Delhi called the Triveni Super Grandmasters Tournament. The joint winners were Kamsky (#8 with a 2650 rating on the July 1990 rating list) and Anand (#19 with a 2610 rating on the same list). I tried to find a full crosstable or participant list for the tournament, but have so far come up empty on such details.
I do know that Judit Polgar (then "only" #108 in the world, though she would crack the top 20 three years later, and the top 10 three years after that) and former champion Mikhail Tal both played. Additionally, it seems this wasn't a tournament exclusively for GMs, but rather one offering lesser lights the chance to play with some "super grandmasters," as Kamsky mentioned in an interview after the tournament that no participants had achieved any grandmaster norms.
Games

Anand-Kamsky, New Delhi 1990
Kamsky-Tal, New Delhi 1990


150: <p>According to the following <a href="http://en.wikipedia.org/wiki/International_Grandmaster" rel="nofollow noreferrer">wiki link</a>,</p>
<blockquote>
<p>The unofficial title, &quot;Super Grandmaster&quot;, is often used by players to refer to those with a 2700+ rating to distinguish the most serious world champion contenders</p>
</blockquote>
<p>When was this term first used and by whom?</p>
 167: <p>As pointed out in the comments, it's nigh impossible to pin down the very first usage, but since you're after a rough idea, I'll get the ball rolling with an initial answer of the oldest definite use I'm aware of; it's from within the last 25 years, so maybe others can improve on this with better answers.</p>

<p>In 1990, there was a tournament held in New Delhi called the <em>Triveni Super Grandmasters Tournament</em>. The joint winners were Kamsky (#8 with a 2650 rating on the <a href="http://www.olimpbase.org/Elo/Elo199007e.html">July 1990</a> rating list) and Anand (#19 with a 2610 rating on the same list). I tried to find a full crosstable or participant list for the tournament, but have so far come up empty on such details.</p>

<p>I do know that Judit Polgar (then "only" #108 in the world, though she would crack the top 20 three years later, and the top 10 three years after that) and former champion Mikhail Tal both played. Additionally, it seems this wasn't a tournament exclusively for GMs, but rather one offering lesser lights the chance to play with some "super grandmasters," as Kamsky mentioned in <a href="http://www.chess-mate.com/mar91in.htm">an interview</a> after the tournament that no participants had achieved any <a href="http://en.wikipedia.org/wiki/Norm_%28chess%29#Grandmaster_norm">grandmaster norms</a>.</p>

<h2>Games</h2>

<ul>
<li><p><a href="http://www.chessgames.com/perl/chessgame?gid=1018142">Anand-Kamsky, New Delhi 1990</a></p></li>
<li><p><a href="http://www.chessgames.com/perl/chessgame?gid=1066553">Kamsky-Tal, New Delhi 1990</a></p></li>
</ul>
