The story
From brown-nose + -er. A metaphorical image: when kissing the buttocks of another person, one's nose is liable to be covered in brown feces.
Etymology from Wiktionary, CC BY-SA 4.0.
brown-noser
Today it means: One who brown-noses (flatters or humors somebody in an obsequious manner for personal gain); one who sucks up; a bootlicker, dickrider, ass-kisser, sycophant. Full definition →