fine thread; thin thread
Refers to a physically slender thread or string. Can be used literally or metaphorically (e.g., a slender connection).
細い糸でボタンを縫い付けた。
I sewed the button on with fine thread.
彼らの友情は細い糸でつながっているようだ。
Their friendship seems to hang by a thin thread.
Compound of 細い (thin) + 糸 (thread). The phrase is transparent and follows standard Japanese noun-modification patterns.