the word "thrush" for the bird has a direct lineage from old english "þrysce", which is of germanic origin. its deeper proto-germanic root is thought to be onomatopoeic, imitating the bird's characteristic chirping song, illustrating a common pattern where bird names arise from their sounds. the application of the same word "thrush" to a fungal infection is a separate, metaphorical development from the 17th century, likely due to a perceived resemblance between the spotted breast of some thrushes and the white patches caused by the oral infection.

the modern english word "thrush" is a simplex word derived from old english "þrysce". it is not productively formed from smaller, recognizable morphemes in modern english.