1. -er a noun-forming suffix, added to nouns to form words designating persons from the object of their occupation or labor (hatter; moonshiner; roofer), or from their place of origin or abode (Icelander; southerner), or designating persons or things from some special characteristic or circumstance (double-decker; fourth-grader; tanker; teenager). When added to verbs, -er1 forms nouns denoting a person, animal or thing that performs or is used in performing the action of the verb ( baker; eye-opener; fertilizer; pointer; teacher; ).
2. -er a nounsuffix occurring in loanwords from French in the Middle English period, most often names of occupations (butcher; carpenter; grocer; mariner; officer), but also other nouns (corner; danger; primer).
3. -er a termination of nouns denoting action or process, occurring orig. and predominantly in loanwords from French or Anglo-French: dinner; rejoinder; remainder.
4. -er a suffix regularly used in forming the comparative degree of adjectives: harder; smaller.
5. -er a suffix regularly used in forming the comparative degree of adverbs: faster.
6. -er a formativeappearing in verbs having frequentative meaning: flicker; flutter; shiver; shudder.
7. -er Chiefly Brit. a suffix that creates informal or jocular mutations of more neutral words, which are typically clipped to a single syllable before application of the suffix, and sometimes subjected to other phonetic alterations: bed-sitter; fresher; rugger; soccer.
Etymology: (prob. modeled on nonagentive uses of -er1; said to have first become current in University College, Oxford, 1875–80)
Definition of '-er'
Webster Dictionary
1. -er
2. -er the termination of many English words, denoting the agent; -- applied either to men or things; as in hater, farmer, heater, grater. At the end of names of places, -er signifies a man of the place; as, Londoner, i. e., London man
3. -er a suffix used to form the comparative degree of adjectives and adverbs; as, warmer, sooner, lat(e)er, earl(y)ier
Definitions of -er 
