Meaning of -ER in English
1》 denoting a person or thing that performs a specified action or activity: farmer.
2》 denoting a person or thing that has a specified attribute or form: two-wheeler.
3》 denoting a person concerned with a specified thing: milliner.
4》 denoting a person belonging to a specified place or group: city-dweller.
OE ~e , of Gmc origin.
■ suffix forming the comparative of adjectives (as in bigger ) and adverbs (as in faster ).
OE suffix -ra (adjectival), -or (adverbial), of Gmc origin.
■ suffix forming nouns used informally, usually by distortion of the root word: footer.
prob. an extended use of ~ 1 ; orig. Rugby School sl., later adopted at Oxford University.
■ suffix forming frequentative verbs such as patter .
OE ~ian , -rian , of Gmc origin.
■ suffix forming nouns:
1》 such as sampler . Compare with -ar 1 . [ending corresponding to L. -aris .]
↘such as butler , danger . [ending corresponding to L. -arius , -arium .]
↘such as border . [ending corresponding (via OFr. -eure ) to L. -atura .]
↘such as laver . See laver 2 . [ending corresponding (via OFr. -eor ) to L. -atorium .]
2》 equivalent to -or 1 .
via OFr. or Anglo-Norman Fr. (see above).
■ suffix chiefly Law (forming nouns) denoting verbal action or a document effecting such action: disclaimer.
from Anglo-Norman Fr. (infin. ending).
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004