noun
full particulars; the whole story; from beginning to end
Used to describe telling or knowing every detail of a matter, leaving nothing out. Often appears in phrases like 一伍一什を話す or 一伍一什を知っている.
彼は事件の一伍一什を警察に話した。
He told the police the whole story of the incident.
その計画の一伍一什を知っているのは社長だけだ。
Only the president knows the full particulars of the plan.
Also means 'the whole story' or 'from beginning to end', but 一伍一什 emphasizes every single detail, while 一部始終 focuses on the sequence of events.
A yojijukugo combining 一 (one) with 伍 (a squad of five) and 什 (a group of ten), originally referring to military units. The phrase came to mean 'every single detail' by extension from the idea of counting every soldier.