Forming nouns and verbs.
[French -ier, from Latin -arius.]
Most words in -eer are nouns denoting a person concerned with or engaged in an activity: auctioneer, mountaineer, mutineer, puppeteer. Others can be verbs denoting concern for or involvement with an activity, sometimes with negative associations: commandeer, domineer, electioneer. Some examples can be both nouns and verbs: engineer (Old French engigneor), profiteer, volunteer. Gazetteer, a geographical index or dictionary, was originally a French word for a journalist who worked on a gazette, a news-sheet.
The suffix is mildly active in the language, usually forming nouns. Some relatively recent examples are imagineer (a creative person, in particular one who devises the attractions in Walt Disney theme parks); rocketeer (a person who works with space rockets; a rocket enthusiast); supermarketeer (organisations that operate supermarkets, or their directors); tabloideer (someone who writes for or manages a tabloid newspaper).
Terms such as overseer, sightseer, and seer come from see plus -er1; veneer is German but from a French root; career in both noun and verb senses derives from French carrière.
See also -ier.