The horrible moment when you realise that you have accidentally done something very slightly wrong which has very bad (usually embarassing) implications for you. This is typically the moment of realisation that you just sent a dirty text message to a close member of family, typically your mother, rather than the intended recipient.
Message: "i'm free this afternoon so come over and fuck me senseless, my parents will be out all day"
Recipient: Mum
Message Sent
- Onosecond occurs here -
by Jimmed September 26, 2006
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An Onosecond is defined as the second after you realise you have done something that is embarrassing or bad.
Examples where an Onosecond can kick in:

Text message contents: A sexy message or a picture of your private parts.
Recipient: a family member.

Seeing a typo in an important essay just after you have sent it.

Accidentally revealing a surprise to someone because you were distracted.
by jackqueslack23 July 29, 2020
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That one moment when you realize you absolutely fucked up and there's no way back, that makes you go: Oh, no.
Dan: You look a bit shaken up, man. What's up?

Todd: I really fucked up today, man. I spilled coffee on my work laptop while working and lost a lot of data. I swear, that was the biggest onosecond I've ever had.
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