When you make up a word in order to rhyme it with an existing one. The term is derived from Dr Seuss an author who wrote children's books like Cat in the Hat and The Grinch.
Dr. Seuss is probably the only person to ever gt away with Seussing because he was awesome like that, but if you do it, you suck.
You may find something like this in a Dr Seuss book.
The animal wrangler fought back the beast with his amazing shcrizzlefangler!
Now, how to use it in a sentence....
Rapper: Yo! rollin' up in my fine ass jeep, bouncin' my head like a verzkizical shmeep!
Poet: Dude, at least try to rhyme, you're just Seussing right now.
The act of making up random words that attempt to, or do, rhyme with previously stated words.
Girl: "What's up?"
Guy: "Not much, tizzuch"
Girl: "The fuck?"
Guy: "What? I like Seussing"