also: せえの · せいの · いっせーの · いっせいの · せいのー
interjection
ready, set, go!; all together now!; heave-ho!
Interjection used to coordinate a simultaneous group action, like lifting a heavy object or starting a race. The exact English equivalent depends on context.
せーの、で持ち上げよう。
On 'ready, set, go,' let's lift it together.
みんなでせーの！
All together now!
Extended form with いっせい; irregular kana usage.
Katakana spelling with a long final vowel; may appear in manga or casual writing.
いっせいに is an adverb meaning 'all at once' or 'simultaneously,' used in descriptive sentences, while せーの is an interjection for coordinating an action.
The origin is uncertain. It may derive from 一斉に (いっせいに, 'all at once') or a contraction of それ (sore) and えい (ei), but there is no consensus.