Etymology
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er 
as a sound of hesitation or uncertainty, attested from mid-19c.
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*er- (2)
Proto-Indo-European root meaning "earth, ground." It forms all or part of: aardvark; aardwolf; earth; earthen; earthy.

It is the hypothetical source of/evidence for its existence is provided by: Old English eorþe "ground, soil, dirt, dry land," Old Norse jörð, Old High German erda, Gothic airþa; Middle Irish -ert "earth."
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headliner (n.)
1891, "one who writes newspaper headlines;" 1896 as "one who stars in a performance;" from headline + -er (1).
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cottager (n.)

"one who lives in a cottage," 1540s, from cottage + -er (1).

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hatter (n.)
late 14c., from hat + -er (1). Their association with madness dates to at least 1837.
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freer (n.)
"one who sets free," c. 1600, from free + -er (1). An Old English word for this was freogend.
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detonator (n.)

"that which causes (something) to explode," 1820, agent noun in Latin form from detonate. For suffix, see -er (1).

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trooper (n.)
1630s, "soldier in a cavalry troop," from troop (n.) + -er (1). Extended to "mounted policeman" (1858, in Australia) then to "state policeman" (U.S.) by 1911.
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Londoner (n.)
"resident or native of London," mid-15c., from London + -er (1). Earlier (late 14c.) was Londenoys, from Anglo-French Londenois.
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