Let me preface this by just informing you that DH is known for his fruity language. He & I have had discussions on the matter since DD was born. Many discussions. He insisted she wouldn't pick it up...
One day when DD was 2, & not long after she'd started talking, we were all 3 going into town, DD was in the buggy. A motorcycle suddenly roared past, unnecessarily noisily, & made DD cry. (She's on the ASD spectrum & struggles with loud noises.) DH commented that the motorcyclist was a knobhead. DD was already crying so we assumed she hadn't heard...
Next day, I took her out in the buggy. As we were waiting by a crossing, 2 old ladies stopped to wait, & admired her curls & dimples. A motorcyclist went past, again, noisy. Clear as a bell & very loudly, she pronounced "knobhead!" I wanted the ground to open up & swallow me. The old ladies, fortunately, saw the funny side. I'm sure I must have been puce with embarrassment. I told DH later, & DD told him, helpfully, what she'd said.
That was the day we started the swear jar.
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We also had artificially loud correcting DD when she said Makka Pakka or caterpillar in public (pronounced Makka pakki & catapaki) Argh!
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Also, there was the time she asked "what's this finger?"
Me: Darling, it's your finger.
DD: Yes, but what is it?
Me: It's your finger, sweetheart.
DD: But mummy, what's this finger? (Getting grumpy now)
Me: Maybe daddy will know?
DH:
We never did find out.