Definition of Big O in English:

Big O

noun

the Big O
informal
  • Orgasm.

    ‘there are ways that you can boost your chances of reaching the big O’
    • ‘Faking the big o on the screen for her starring role in "When Harry Met Sally," Meg Ryan got her start on "As the World Turns."’
    • ‘From Charlotte's dalliances with a pink vibrator called The Rabbit to Samantha's short-lived horror at losing her mojo, the big O has been demystified in a way that even how-to manual The Joy Of Sex didn't manage.’
    • ‘When it comes to the big O I can't judge anyone for going out of their way to guarantee results.’
    • ‘One in three women reported never or infrequently mouthing the big O during sexual intercourse.’
    • ‘As other women's magazines focus almost exclusively on the other big O - the orgasm - "O" unabashedly features all things Oprah.’
    • ‘I mean, come on - a guy who listened and supported you and gave you your first big O?’
    • ‘Ever wondered what's going on when you reach the big O?’

Origin

1960s: from big + the initial letter of orgasm.