To make a high-pitched sound like a horse.
Squinnied can make people think of horses and funny noises.
Examples:
The horse squinnied loudly in the field.
I heard a horse squinnied in the distance.
She squinnied at the funny joke.
The cat squinnied like a horse, it was very funny!
The little girl squinnied when she saw the clown.
The horse squinnied and everyone laughed.