preen (v.)

late 14c., of a person, "to trim, to dress up," probably a variation of Middle English proynen, proinen, of a bird, "trim the feather with the beak;" of a person, "to dress or groom oneself carefully" (see prune (v.)). Middle English prene "to pin, pierce, fasten with a pin" probably influenced the form of this word. It is from Old English preon, a general Germanic word (compare Dutch priemen, Low German prünen, East Frisian prinen).

In English, the use in reference to a bird, "to trim the feathers with the beak," is from late 15c. Because of the late medieval popularity of falconry, bird activities were more closely observed and words for them were more precise in English than today.

Youre hawke proynith and not pikith and she prenyth not bot whan she begynnyth at hir leggys, and fetcheth moystour like oyle at hir taill. ["Book of St. Albans," 1486]

Preening as a present-participle adjective meaning "proud, self-confident" is by 1903.

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Definitions of preen from WordNet

preen (v.)
clean with one's bill;
The birds preened
Synonyms: plume
preen (v.)
pride or congratulate (oneself) for an achievement;
Synonyms: congratulate
preen (v.)
dress or groom with elaborate care;
Synonyms: primp / plume / dress