I first heard of Slim Jims at the same time they rose to national prominence in the 90's when they used Machoman Randy Savage to paint what's essentially, uh, mechanically separated chicken jerky, as a snack so extreme that it made a cracking whip sound when you bit into it.
I think it was the lack of the sound effect that disappointed me most about Slim Jims.
Well, turns out this in-your-face macho snack food angle had been in effect at least as early as the mid-70's, as this ad shows:
Pretty clever ad name, huh? I mean, sure it might be actually more effective to say that Slim Jims are so extreme even werewolves would eat them (which it actually implies at the end of the ad), and the final slogan of "a little less than a meal, a little more than a snack" could be worded less self-deprecatingly, but when most ads of these period are super-dry, super-quaint, or super queer, lines such as "...you can find it at the grocery store. Or at a lot of the places where you've been getting those sissy snacks" are a breath of fresh, modern air.
Of course, in a few decades time we get to the point where Machoman is yelling and whipcracking meat at your face. So you can decide for yourself whether we've advanced as a smoked-beef-snack eating species.
From Amazing Spider-Man #148, September 1975.