Refers to the initial stage or the very beginning of something, often implying a small opening that leads to a larger development. Common in phrases like 糸口を見つける (find a starting point) or 糸口をつかむ (get a foothold).
Used when a small piece of information or a trace provides a way into understanding or solving something. Often overlaps with sense 1, but emphasizes the clue-like nature.
Literal meaning: the end of a thread. This is the original, concrete sense, but it is rarely used in modern everyday conversation. Mostly encountered in explanations of the word's origin or in literary contexts.
