substitute

"What was that?" I asked my grandson.
"A substitute," he answered.
"Why did you say substitute?" I asked wonderingly.
"Because I was embarrassed to say toot," he explained.
"Do you know what a substitute is? I queried, suppressing a sudden desire to collapse in laughter.
"Yes," he nodded. "It's something that takes the place of something else."
And of course, that's what all slang is. A substitute.