Definition of a kick in the pants (or up the backside) in US English:

a kick in the pants (or up the backside)

phrase

informal
  • An unwelcome surprise that prompts or forces fresh effort.

    ‘the competition will be healthy, but we needed a kick in the pants’
    • ‘It is easy to dismiss Ivanov, alongside Chekhov's other plays, as being full of melancholy middle class moaners who need a kick up the backside.’
    • ‘Failing to qualify for the final two rounds of the St Andrews Links Trophy last week gave me a kick up the backside which I fully deserved.’
    • ‘We had a bit of a kick up the backside at half time, which we deserved, and that spurred us on.’
    • ‘She is there for me when things go wrong - and when I need a kick up the backside.’
    • ‘Sometimes people need empathy; sometimes they need a kick in the pants.’
    • ‘He is a very good coach in every respect, the first guy to give you a kick up the backside but also the first to give you a pat on the back when he feels you deserve it.’
    • ‘Today, if I was the president, I would dismiss the coach and line the players up against a wall and give them all a kick up the backside.’
    • ‘Stuttgart gave me a kick up the backside, and I re-focused and came out better last year - it was what I needed.’
    • ‘He strikes me as a guy who needs a lot of love - maybe a lot of love and a kick in the pants.’
    • ‘We think we've got a good team but, every now and then, we could use a kick up the backside.’