An overall, coverall, over all, or dungarees, is a type of garment which is usually used as protective clothing when working. Some people call an overall a "pair of overalls" by analogy with "pair of trousers

Trousers

Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...

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The 1989 issue of the Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

 lists:

• The word "overall" (as garment):
  • First in 1792 as "overalls" or "overall trousers" = "trousers
    Trousers
    Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...
    
     worn outside the normal trousers to protect them" (from which the "bib
    Bib (garment)
    A bib is a garment worn hanging from the neck on the chest to protect clothing from spilling.The word, reported in English since 1580, stems from a verb bibben "to drink" , from Latin bibere, either because it was worn while drinking or because it "soaked up" spills.Bibs are frequently used by...
    
    -and-brace
    Suspenders
    Suspenders or braces are fabric or leather straps worn over the shoulders to hold up trousers. Straps may be elasticated, either entirely or only at attachment ends and most straps are of woven cloth forming an X or Y shape at the back. Braces are typically attached to trousers with buttons...
    
    " use).
  • First in 1815 as "overall" = "any outermost coat or cloak", with a long list of examples, which do not show when "overall" began to mean "boilersuit".
• The word "boilersuit" first on 28 October 1928 in the Sunday Express newspaper.

The first mention of boilersuits known here is in a special rule for manufacturing explosive

Explosive material

An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...

s, laid down in 1891: "Overall suits and head covering shall be supplied to all workers…"

The one-piece work overall arrived in 1891-1916, in tough cotton or in linen, to fit over a shirt or vest and trousers.