There are a lot of cool answers to this question and they are all correct depending on your definition of which era you are talking about.
One answer is a toss up between a stick and a rock, as both were tools with multiple purposes.
Another good answer is the Roman tool posted above.
There are also a series here called "Before there was Leatherman" in which out own multitools historian J-Sews looks at plier based multitools that were available prior to Tim Leatherman's 12 in 1 tool being released in the early 80's.
Undoubtedly the first multitool of the modern era is that 12 in 1 tool, which was eventually named the PST or Pocket Survival Tool.
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